[dev.garbage] all: merge dev.cc into dev.garbage

[gostls13.git] / src / runtime / proc1.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import "unsafe"

var (
        m0 m
        g0 g
)

// Goroutine scheduler
// The scheduler's job is to distribute ready-to-run goroutines over worker threads.
//
// The main concepts are:
// G - goroutine.
// M - worker thread, or machine.
// P - processor, a resource that is required to execute Go code.
//     M must have an associated P to execute Go code, however it can be
//     blocked or in a syscall w/o an associated P.
//
// Design doc at http://golang.org/s/go11sched.

const (
        // Number of goroutine ids to grab from sched.goidgen to local per-P cache at once.
        // 16 seems to provide enough amortization, but other than that it's mostly arbitrary number.
        _GoidCacheBatch = 16
)

/*
SchedT  sched;
int32   gomaxprocs;
uint32  needextram;
bool    iscgo;
M       m0;
G       g0;     // idle goroutine for m0
G*      lastg;
M*      allm;
M*      extram;
P*      allp[MaxGomaxprocs+1];
int8*   goos;
int32   ncpu;
int32   newprocs;

Mutex allglock; // the following vars are protected by this lock or by stoptheworld
G**     allg;
Slice   allgs;
uintptr allglen;
ForceGCState    forcegc;

void mstart(void);
static void runqput(P*, G*);
static G* runqget(P*);
static bool runqputslow(P*, G*, uint32, uint32);
static G* runqsteal(P*, P*);
static void mput(M*);
static M* mget(void);
static void mcommoninit(M*);
static void schedule(void);
static void procresize(int32);
static void acquirep(P*);
static P* releasep(void);
static void newm(void(*)(void), P*);
static void stopm(void);
static void startm(P*, bool);
static void handoffp(P*);
static void wakep(void);
static void stoplockedm(void);
static void startlockedm(G*);
static void sysmon(void);
static uint32 retake(int64);
static void incidlelocked(int32);
static void checkdead(void);
static void exitsyscall0(G*);
void park_m(G*);
static void goexit0(G*);
static void gfput(P*, G*);
static G* gfget(P*);
static void gfpurge(P*);
static void globrunqput(G*);
static void globrunqputbatch(G*, G*, int32);
static G* globrunqget(P*, int32);
static P* pidleget(void);
static void pidleput(P*);
static void injectglist(G*);
static bool preemptall(void);
static bool preemptone(P*);
static bool exitsyscallfast(void);
static bool haveexperiment(int8*);
void allgadd(G*);
static void dropg(void);

extern String buildVersion;
*/

// The bootstrap sequence is:
//
//      call osinit
//      call schedinit
//      make & queue new G
//      call runtime·mstart
//
// The new G calls runtime·main.
func schedinit() {
        // raceinit must be the first call to race detector.
        // In particular, it must be done before mallocinit below calls racemapshadow.
        _g_ := getg()
        if raceenabled {
                _g_.racectx = raceinit()
        }

        sched.maxmcount = 10000

        tracebackinit()
        symtabinit()
        stackinit()
        mallocinit()
        mcommoninit(_g_.m)

        goargs()
        goenvs()
        parsedebugvars()
        gcinit()

        sched.lastpoll = uint64(nanotime())
        procs := 1
        if n := goatoi(gogetenv("GOMAXPROCS")); n > 0 {
                if n > _MaxGomaxprocs {
                        n = _MaxGomaxprocs
                }
                procs = n
        }
        procresize(int32(procs))

        if buildVersion == "" {
                // Condition should never trigger.  This code just serves
                // to ensure runtime·buildVersion is kept in the resulting binary.
                buildVersion = "unknown"
        }
}

func newsysmon() {
        _newm(sysmon, nil)
}

func dumpgstatus(gp *g) {
        _g_ := getg()
        print("runtime: gp: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
        print("runtime:  g:  g=", _g_, ", goid=", _g_.goid, ",  g->atomicstatus=", readgstatus(_g_), "\n")
}

func checkmcount() {
        // sched lock is held
        if sched.mcount > sched.maxmcount {
                print("runtime: program exceeds ", sched.maxmcount, "-thread limit\n")
                gothrow("thread exhaustion")
        }
}

func mcommoninit(mp *m) {
        _g_ := getg()

        // g0 stack won't make sense for user (and is not necessary unwindable).
        if _g_ != _g_.m.g0 {
                callers(1, &mp.createstack[0], len(mp.createstack))
        }

        mp.fastrand = 0x49f6428a + uint32(mp.id) + uint32(cputicks())
        if mp.fastrand == 0 {
                mp.fastrand = 0x49f6428a
        }

        lock(&sched.lock)
        mp.id = sched.mcount
        sched.mcount++
        checkmcount()
        mpreinit(mp)
        if mp.gsignal != nil {
                mp.gsignal.stackguard1 = mp.gsignal.stack.lo + _StackGuard
        }

        // Add to allm so garbage collector doesn't free g->m
        // when it is just in a register or thread-local storage.
        mp.alllink = allm

        // NumCgoCall() iterates over allm w/o schedlock,
        // so we need to publish it safely.
        atomicstorep(unsafe.Pointer(&allm), unsafe.Pointer(mp))
        unlock(&sched.lock)
}

// Mark gp ready to run.
func ready(gp *g) {
        status := readgstatus(gp)

        // Mark runnable.
        _g_ := getg()
        _g_.m.locks++ // disable preemption because it can be holding p in a local var
        if status&^_Gscan != _Gwaiting {
                dumpgstatus(gp)
                gothrow("bad g->status in ready")
        }

        // status is Gwaiting or Gscanwaiting, make Grunnable and put on runq
        casgstatus(gp, _Gwaiting, _Grunnable)
        runqput(_g_.m.p, gp)
        if atomicload(&sched.npidle) != 0 && atomicload(&sched.nmspinning) == 0 { // TODO: fast atomic
                wakep()
        }
        _g_.m.locks--
        if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
                _g_.stackguard0 = stackPreempt
        }
}

func gcprocs() int32 {
        // Figure out how many CPUs to use during GC.
        // Limited by gomaxprocs, number of actual CPUs, and MaxGcproc.
        lock(&sched.lock)
        n := gomaxprocs
        if n > ncpu {
                n = ncpu
        }
        if n > _MaxGcproc {
                n = _MaxGcproc
        }
        if n > sched.nmidle+1 { // one M is currently running
                n = sched.nmidle + 1
        }
        unlock(&sched.lock)
        return n
}

func needaddgcproc() bool {
        lock(&sched.lock)
        n := gomaxprocs
        if n > ncpu {
                n = ncpu
        }
        if n > _MaxGcproc {
                n = _MaxGcproc
        }
        n -= sched.nmidle + 1 // one M is currently running
        unlock(&sched.lock)
        return n > 0
}

func helpgc(nproc int32) {
        _g_ := getg()
        lock(&sched.lock)
        pos := 0
        for n := int32(1); n < nproc; n++ { // one M is currently running
                if allp[pos].mcache == _g_.m.mcache {
                        pos++
                }
                mp := mget()
                if mp == nil {
                        gothrow("gcprocs inconsistency")
                }
                mp.helpgc = n
                mp.mcache = allp[pos].mcache
                pos++
                notewakeup(&mp.park)
        }
        unlock(&sched.lock)
}

// Similar to stoptheworld but best-effort and can be called several times.
// There is no reverse operation, used during crashing.
// This function must not lock any mutexes.
func freezetheworld() {
        if gomaxprocs == 1 {
                return
        }
        // stopwait and preemption requests can be lost
        // due to races with concurrently executing threads,
        // so try several times
        for i := 0; i < 5; i++ {
                // this should tell the scheduler to not start any new goroutines
                sched.stopwait = 0x7fffffff
                atomicstore(&sched.gcwaiting, 1)
                // this should stop running goroutines
                if !preemptall() {
                        break // no running goroutines
                }
                usleep(1000)
        }
        // to be sure
        usleep(1000)
        preemptall()
        usleep(1000)
}

func isscanstatus(status uint32) bool {
        if status == _Gscan {
                gothrow("isscanstatus: Bad status Gscan")
        }
        return status&_Gscan == _Gscan
}

// All reads and writes of g's status go through readgstatus, casgstatus
// castogscanstatus, casfrom_Gscanstatus.
//go:nosplit
func readgstatus(gp *g) uint32 {
        return atomicload(&gp.atomicstatus)
}

// The Gscanstatuses are acting like locks and this releases them.
// If it proves to be a performance hit we should be able to make these
// simple atomic stores but for now we are going to throw if
// we see an inconsistent state.
func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
        success := false

        // Check that transition is valid.
        switch oldval {
        case _Gscanrunnable,
                _Gscanwaiting,
                _Gscanrunning,
                _Gscansyscall:
                if newval == oldval&^_Gscan {
                        success = cas(&gp.atomicstatus, oldval, newval)
                }
        case _Gscanenqueue:
                if newval == _Gwaiting {
                        success = cas(&gp.atomicstatus, oldval, newval)
                }
        }
        if !success {
                print("runtime: casfrom_Gscanstatus failed gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
                dumpgstatus(gp)
                gothrow("casfrom_Gscanstatus: gp->status is not in scan state")
        }
}

// This will return false if the gp is not in the expected status and the cas fails.
// This acts like a lock acquire while the casfromgstatus acts like a lock release.
func castogscanstatus(gp *g, oldval, newval uint32) bool {
        switch oldval {
        case _Grunnable,
                _Gwaiting,
                _Gsyscall:
                if newval == oldval|_Gscan {
                        return cas(&gp.atomicstatus, oldval, newval)
                }
        case _Grunning:
                if newval == _Gscanrunning || newval == _Gscanenqueue {
                        return cas(&gp.atomicstatus, oldval, newval)
                }
        }
        print("runtime: castogscanstatus oldval=", hex(oldval), " newval=", hex(newval), "\n")
        gothrow("castogscanstatus")
        panic("not reached")
}

// If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus
// and casfrom_Gscanstatus instead.
// casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that
// put it in the Gscan state is finished.
//go:nosplit
func casgstatus(gp *g, oldval, newval uint32) {
        if (oldval&_Gscan != 0) || (newval&_Gscan != 0) || oldval == newval {
                systemstack(func() {
                        print("casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n")
                        gothrow("casgstatus: bad incoming values")
                })
        }

        // loop if gp->atomicstatus is in a scan state giving
        // GC time to finish and change the state to oldval.
        for !cas(&gp.atomicstatus, oldval, newval) {
        }
}

// stopg ensures that gp is stopped at a GC safe point where its stack can be scanned
// or in the context of a moving collector the pointers can be flipped from pointing
// to old object to pointing to new objects.
// If stopg returns true, the caller knows gp is at a GC safe point and will remain there until
// the caller calls restartg.
// If stopg returns false, the caller is not responsible for calling restartg. This can happen
// if another thread, either the gp itself or another GC thread is taking the responsibility
// to do the GC work related to this thread.
func stopg(gp *g) bool {
        for {
                if gp.gcworkdone {
                        return false
                }

                switch s := readgstatus(gp); s {
                default:
                        dumpgstatus(gp)
                        gothrow("stopg: gp->atomicstatus is not valid")

                case _Gdead:
                        return false

                case _Gcopystack:
                        // Loop until a new stack is in place.

                case _Grunnable,
                        _Gsyscall,
                        _Gwaiting:
                        // Claim goroutine by setting scan bit.
                        if !castogscanstatus(gp, s, s|_Gscan) {
                                break
                        }
                        // In scan state, do work.
                        gcphasework(gp)
                        return true

                case _Gscanrunnable,
                        _Gscanwaiting,
                        _Gscansyscall:
                        // Goroutine already claimed by another GC helper.
                        return false

                case _Grunning:
                        if gcphase == _GCscan {
                                // Running routines not scanned during
                                // GCscan phase, we only scan non-running routines.
                                gp.gcworkdone = true
                                return false
                        }

                        // Claim goroutine, so we aren't racing with a status
                        // transition away from Grunning.
                        if !castogscanstatus(gp, _Grunning, _Gscanrunning) {
                                break
                        }

                        // Mark gp for preemption.
                        if !gp.gcworkdone {
                                gp.preemptscan = true
                                gp.preempt = true
                                gp.stackguard0 = stackPreempt
                        }

                        // Unclaim.
                        casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)
                        return false
                }
        }
}

// The GC requests that this routine be moved from a scanmumble state to a mumble state.
func restartg(gp *g) {
        s := readgstatus(gp)
        switch s {
        default:
                dumpgstatus(gp)
                gothrow("restartg: unexpected status")

        case _Gdead:
                // ok

        case _Gscanrunnable,
                _Gscanwaiting,
                _Gscansyscall:
                casfrom_Gscanstatus(gp, s, s&^_Gscan)

        // Scan is now completed.
        // Goroutine now needs to be made runnable.
        // We put it on the global run queue; ready blocks on the global scheduler lock.
        case _Gscanenqueue:
                casfrom_Gscanstatus(gp, _Gscanenqueue, _Gwaiting)
                if gp != getg().m.curg {
                        gothrow("processing Gscanenqueue on wrong m")
                }
                dropg()
                ready(gp)
        }
}

func stopscanstart(gp *g) {
        _g_ := getg()
        if _g_ == gp {
                gothrow("GC not moved to G0")
        }
        if stopg(gp) {
                if !isscanstatus(readgstatus(gp)) {
                        dumpgstatus(gp)
                        gothrow("GC not in scan state")
                }
                restartg(gp)
        }
}

// Runs on g0 and does the actual work after putting the g back on the run queue.
func mquiesce(gpmaster *g) {
        // enqueue the calling goroutine.
        restartg(gpmaster)

        activeglen := len(allgs)
        for i := 0; i < activeglen; i++ {
                gp := allgs[i]
                if readgstatus(gp) == _Gdead {
                        gp.gcworkdone = true // noop scan.
                } else {
                        gp.gcworkdone = false
                }
                stopscanstart(gp)
        }

        // Check that the G's gcwork (such as scanning) has been done. If not do it now.
        // You can end up doing work here if the page trap on a Grunning Goroutine has
        // not been sprung or in some race situations. For example a runnable goes dead
        // and is started up again with a gp->gcworkdone set to false.
        for i := 0; i < activeglen; i++ {
                gp := allgs[i]
                for !gp.gcworkdone {
                        status := readgstatus(gp)
                        if status == _Gdead {
                                //do nothing, scan not needed.
                                gp.gcworkdone = true // scan is a noop
                                break
                        }
                        if status == _Grunning && gp.stackguard0 == uintptr(stackPreempt) && notetsleep(&sched.stopnote, 100*1000) { // nanosecond arg
                                noteclear(&sched.stopnote)
                        } else {
                                stopscanstart(gp)
                        }
                }
        }

        for i := 0; i < activeglen; i++ {
                gp := allgs[i]
                status := readgstatus(gp)
                if isscanstatus(status) {
                        print("mstopandscang:bottom: post scan bad status gp=", gp, " has status ", hex(status), "\n")
                        dumpgstatus(gp)
                }
                if !gp.gcworkdone && status != _Gdead {
                        print("mstopandscang:bottom: post scan gp=", gp, "->gcworkdone still false\n")
                        dumpgstatus(gp)
                }
        }

        schedule() // Never returns.
}

// quiesce moves all the goroutines to a GC safepoint which for now is a at preemption point.
// If the global gcphase is GCmark quiesce will ensure that all of the goroutine's stacks
// have been scanned before it returns.
func quiesce(mastergp *g) {
        castogscanstatus(mastergp, _Grunning, _Gscanenqueue)
        // Now move this to the g0 (aka m) stack.
        // g0 will potentially scan this thread and put mastergp on the runqueue
        mcall(mquiesce)
}

// This is used by the GC as well as the routines that do stack dumps. In the case
// of GC all the routines can be reliably stopped. This is not always the case
// when the system is in panic or being exited.
func stoptheworld() {
        _g_ := getg()

        // If we hold a lock, then we won't be able to stop another M
        // that is blocked trying to acquire the lock.
        if _g_.m.locks > 0 {
                gothrow("stoptheworld: holding locks")
        }

        lock(&sched.lock)
        sched.stopwait = gomaxprocs
        atomicstore(&sched.gcwaiting, 1)
        preemptall()
        // stop current P
        _g_.m.p.status = _Pgcstop // Pgcstop is only diagnostic.
        sched.stopwait--
        // try to retake all P's in Psyscall status
        for i := 0; i < int(gomaxprocs); i++ {
                p := allp[i]
                s := p.status
                if s == _Psyscall && cas(&p.status, s, _Pgcstop) {
                        sched.stopwait--
                }
        }
        // stop idle P's
        for {
                p := pidleget()
                if p == nil {
                        break
                }
                p.status = _Pgcstop
                sched.stopwait--
        }
        wait := sched.stopwait > 0
        unlock(&sched.lock)

        // wait for remaining P's to stop voluntarily
        if wait {
                for {
                        // wait for 100us, then try to re-preempt in case of any races
                        if notetsleep(&sched.stopnote, 100*1000) {
                                noteclear(&sched.stopnote)
                                break
                        }
                        preemptall()
                }
        }
        if sched.stopwait != 0 {
                gothrow("stoptheworld: not stopped")
        }
        for i := 0; i < int(gomaxprocs); i++ {
                p := allp[i]
                if p.status != _Pgcstop {
                        gothrow("stoptheworld: not stopped")
                }
        }
}

func mhelpgc() {
        _g_ := getg()
        _g_.m.helpgc = -1
}

func starttheworld() {
        _g_ := getg()

        _g_.m.locks++        // disable preemption because it can be holding p in a local var
        gp := netpoll(false) // non-blocking
        injectglist(gp)
        add := needaddgcproc()
        lock(&sched.lock)
        if newprocs != 0 {
                procresize(newprocs)
                newprocs = 0
        } else {
                procresize(gomaxprocs)
        }
        sched.gcwaiting = 0

        var p1 *p
        for {
                p := pidleget()
                if p == nil {
                        break
                }
                // procresize() puts p's with work at the beginning of the list.
                // Once we reach a p without a run queue, the rest don't have one either.
                if p.runqhead == p.runqtail {
                        pidleput(p)
                        break
                }
                p.m = mget()
                p.link = p1
                p1 = p
        }
        if sched.sysmonwait != 0 {
                sched.sysmonwait = 0
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)

        for p1 != nil {
                p := p1
                p1 = p1.link
                if p.m != nil {
                        mp := p.m
                        p.m = nil
                        if mp.nextp != nil {
                                gothrow("starttheworld: inconsistent mp->nextp")
                        }
                        mp.nextp = p
                        notewakeup(&mp.park)
                } else {
                        // Start M to run P.  Do not start another M below.
                        _newm(nil, p)
                        add = false
                }
        }

        if add {
                // If GC could have used another helper proc, start one now,
                // in the hope that it will be available next time.
                // It would have been even better to start it before the collection,
                // but doing so requires allocating memory, so it's tricky to
                // coordinate.  This lazy approach works out in practice:
                // we don't mind if the first couple gc rounds don't have quite
                // the maximum number of procs.
                _newm(mhelpgc, nil)
        }
        _g_.m.locks--
        if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
                _g_.stackguard0 = stackPreempt
        }
}

// Called to start an M.
//go:nosplit
func mstart() {
        _g_ := getg()

        if _g_.stack.lo == 0 {
                // Initialize stack bounds from system stack.
                // Cgo may have left stack size in stack.hi.
                size := _g_.stack.hi
                if size == 0 {
                        size = 8192
                }
                _g_.stack.hi = uintptr(noescape(unsafe.Pointer(&size)))
                _g_.stack.lo = _g_.stack.hi - size + 1024
        }
        // Initialize stack guards so that we can start calling
        // both Go and C functions with stack growth prologues.
        _g_.stackguard0 = _g_.stack.lo + _StackGuard
        _g_.stackguard1 = _g_.stackguard0
        mstart1()
}

func mstart1() {
        _g_ := getg()

        if _g_ != _g_.m.g0 {
                gothrow("bad runtime·mstart")
        }

        // Record top of stack for use by mcall.
        // Once we call schedule we're never coming back,
        // so other calls can reuse this stack space.
        gosave(&_g_.m.g0.sched)
        _g_.m.g0.sched.pc = ^uintptr(0) // make sure it is never used
        asminit()
        minit()

        // Install signal handlers; after minit so that minit can
        // prepare the thread to be able to handle the signals.
        if _g_.m == &m0 {
                initsig()
        }

        if _g_.m.mstartfn != nil {
                fn := *(*func())(unsafe.Pointer(&_g_.m.mstartfn))
                fn()
        }

        if _g_.m.helpgc != 0 {
                _g_.m.helpgc = 0
                stopm()
        } else if _g_.m != &m0 {
                acquirep(_g_.m.nextp)
                _g_.m.nextp = nil
        }
        schedule()

        // TODO(brainman): This point is never reached, because scheduler
        // does not release os threads at the moment. But once this path
        // is enabled, we must remove our seh here.
}

// When running with cgo, we call _cgo_thread_start
// to start threads for us so that we can play nicely with
// foreign code.
var cgoThreadStart unsafe.Pointer

type cgothreadstart struct {
        g   *g
        tls *uint64
        fn  unsafe.Pointer
}

// Allocate a new m unassociated with any thread.
// Can use p for allocation context if needed.
func allocm(_p_ *p) *m {
        _g_ := getg()
        _g_.m.locks++ // disable GC because it can be called from sysmon
        if _g_.m.p == nil {
                acquirep(_p_) // temporarily borrow p for mallocs in this function
        }
        mp := newM()
        mcommoninit(mp)

        // In case of cgo or Solaris, pthread_create will make us a stack.
        // Windows and Plan 9 will layout sched stack on OS stack.
        if iscgo || GOOS == "solaris" || GOOS == "windows" || GOOS == "plan9" {
                mp.g0 = malg(-1)
        } else {
                mp.g0 = malg(8192)
        }
        mp.g0.m = mp

        if _p_ == _g_.m.p {
                releasep()
        }
        _g_.m.locks--
        if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
                _g_.stackguard0 = stackPreempt
        }

        return mp
}

func allocg() *g {
        return newG()
}

// needm is called when a cgo callback happens on a
// thread without an m (a thread not created by Go).
// In this case, needm is expected to find an m to use
// and return with m, g initialized correctly.
// Since m and g are not set now (likely nil, but see below)
// needm is limited in what routines it can call. In particular
// it can only call nosplit functions (textflag 7) and cannot
// do any scheduling that requires an m.
//
// In order to avoid needing heavy lifting here, we adopt
// the following strategy: there is a stack of available m's
// that can be stolen. Using compare-and-swap
// to pop from the stack has ABA races, so we simulate
// a lock by doing an exchange (via casp) to steal the stack
// head and replace the top pointer with MLOCKED (1).
// This serves as a simple spin lock that we can use even
// without an m. The thread that locks the stack in this way
// unlocks the stack by storing a valid stack head pointer.
//
// In order to make sure that there is always an m structure
// available to be stolen, we maintain the invariant that there
// is always one more than needed. At the beginning of the
// program (if cgo is in use) the list is seeded with a single m.
// If needm finds that it has taken the last m off the list, its job
// is - once it has installed its own m so that it can do things like
// allocate memory - to create a spare m and put it on the list.
//
// Each of these extra m's also has a g0 and a curg that are
// pressed into service as the scheduling stack and current
// goroutine for the duration of the cgo callback.
//
// When the callback is done with the m, it calls dropm to
// put the m back on the list.
//go:nosplit
func needm(x byte) {
        if needextram != 0 {
                // Can happen if C/C++ code calls Go from a global ctor.
                // Can not throw, because scheduler is not initialized yet.
                // XXX
                // write(2, unsafe.Pointer("fatal error: cgo callback before cgo call\n"), sizeof("fatal error: cgo callback before cgo call\n") - 1)
                exit(1)
        }

        // Lock extra list, take head, unlock popped list.
        // nilokay=false is safe here because of the invariant above,
        // that the extra list always contains or will soon contain
        // at least one m.
        mp := lockextra(false)

        // Set needextram when we've just emptied the list,
        // so that the eventual call into cgocallbackg will
        // allocate a new m for the extra list. We delay the
        // allocation until then so that it can be done
        // after exitsyscall makes sure it is okay to be
        // running at all (that is, there's no garbage collection
        // running right now).
        mp.needextram = mp.schedlink == nil
        unlockextra(mp.schedlink)

        // Install g (= m->g0) and set the stack bounds
        // to match the current stack. We don't actually know
        // how big the stack is, like we don't know how big any
        // scheduling stack is, but we assume there's at least 32 kB,
        // which is more than enough for us.
        setg(mp.g0)
        _g_ := getg()
        _g_.stack.hi = uintptr(noescape(unsafe.Pointer(&x))) + 1024
        _g_.stack.lo = uintptr(noescape(unsafe.Pointer(&x))) - 32*1024
        _g_.stackguard0 = _g_.stack.lo + _StackGuard

        // Initialize this thread to use the m.
        asminit()
        minit()
}

// newextram allocates an m and puts it on the extra list.
// It is called with a working local m, so that it can do things
// like call schedlock and allocate.
func newextram() {
        // Create extra goroutine locked to extra m.
        // The goroutine is the context in which the cgo callback will run.
        // The sched.pc will never be returned to, but setting it to
        // goexit makes clear to the traceback routines where
        // the goroutine stack ends.
        mp := allocm(nil)
        gp := malg(4096)
        gp.sched.pc = funcPC(goexit) + _PCQuantum
        gp.sched.sp = gp.stack.hi
        gp.sched.sp -= 4 * regSize // extra space in case of reads slightly beyond frame
        gp.sched.lr = 0
        gp.sched.g = gp
        gp.syscallpc = gp.sched.pc
        gp.syscallsp = gp.sched.sp
        // malg returns status as Gidle, change to Gsyscall before adding to allg
        // where GC will see it.
        casgstatus(gp, _Gidle, _Gsyscall)
        gp.m = mp
        mp.curg = gp
        mp.locked = _LockInternal
        mp.lockedg = gp
        gp.lockedm = mp
        gp.goid = int64(xadd64(&sched.goidgen, 1))
        if raceenabled {
                gp.racectx = racegostart(funcPC(newextram))
        }
        // put on allg for garbage collector
        allgadd(gp)

        // Add m to the extra list.
        mnext := lockextra(true)
        mp.schedlink = mnext
        unlockextra(mp)
}

// dropm is called when a cgo callback has called needm but is now
// done with the callback and returning back into the non-Go thread.
// It puts the current m back onto the extra list.
//
// The main expense here is the call to signalstack to release the
// m's signal stack, and then the call to needm on the next callback
// from this thread. It is tempting to try to save the m for next time,
// which would eliminate both these costs, but there might not be
// a next time: the current thread (which Go does not control) might exit.
// If we saved the m for that thread, there would be an m leak each time
// such a thread exited. Instead, we acquire and release an m on each
// call. These should typically not be scheduling operations, just a few
// atomics, so the cost should be small.
//
// TODO(rsc): An alternative would be to allocate a dummy pthread per-thread
// variable using pthread_key_create. Unlike the pthread keys we already use
// on OS X, this dummy key would never be read by Go code. It would exist
// only so that we could register at thread-exit-time destructor.
// That destructor would put the m back onto the extra list.
// This is purely a performance optimization. The current version,
// in which dropm happens on each cgo call, is still correct too.
// We may have to keep the current version on systems with cgo
// but without pthreads, like Windows.
func dropm() {
        // Undo whatever initialization minit did during needm.
        unminit()

        // Clear m and g, and return m to the extra list.
        // After the call to setmg we can only call nosplit functions.
        mp := getg().m
        setg(nil)

        mnext := lockextra(true)
        mp.schedlink = mnext
        unlockextra(mp)
}

var extram uintptr

// lockextra locks the extra list and returns the list head.
// The caller must unlock the list by storing a new list head
// to extram. If nilokay is true, then lockextra will
// return a nil list head if that's what it finds. If nilokay is false,
// lockextra will keep waiting until the list head is no longer nil.
//go:nosplit
func lockextra(nilokay bool) *m {
        const locked = 1

        for {
                old := atomicloaduintptr(&extram)
                if old == locked {
                        yield := osyield
                        yield()
                        continue
                }
                if old == 0 && !nilokay {
                        usleep(1)
                        continue
                }
                if casuintptr(&extram, old, locked) {
                        return (*m)(unsafe.Pointer(old))
                }
                yield := osyield
                yield()
                continue
        }
}

//go:nosplit
func unlockextra(mp *m) {
        atomicstoreuintptr(&extram, uintptr(unsafe.Pointer(mp)))
}

// Create a new m.  It will start off with a call to fn, or else the scheduler.
func _newm(fn func(), _p_ *p) {
        mp := allocm(_p_)
        mp.nextp = _p_
        mp.mstartfn = *(*unsafe.Pointer)(unsafe.Pointer(&fn))

        if iscgo {
                var ts cgothreadstart
                if _cgo_thread_start == nil {
                        gothrow("_cgo_thread_start missing")
                }
                ts.g = mp.g0
                ts.tls = (*uint64)(unsafe.Pointer(&mp.tls[0]))
                ts.fn = unsafe.Pointer(funcPC(mstart))
                asmcgocall(_cgo_thread_start, unsafe.Pointer(&ts))
                return
        }
        newosproc(mp, unsafe.Pointer(mp.g0.stack.hi))
}

// Stops execution of the current m until new work is available.
// Returns with acquired P.
func stopm() {
        _g_ := getg()

        if _g_.m.locks != 0 {
                gothrow("stopm holding locks")
        }
        if _g_.m.p != nil {
                gothrow("stopm holding p")
        }
        if _g_.m.spinning {
                _g_.m.spinning = false
                xadd(&sched.nmspinning, -1)
        }

retry:
        lock(&sched.lock)
        mput(_g_.m)
        unlock(&sched.lock)
        notesleep(&_g_.m.park)
        noteclear(&_g_.m.park)
        if _g_.m.helpgc != 0 {
                gchelper()
                _g_.m.helpgc = 0
                _g_.m.mcache = nil
                goto retry
        }
        acquirep(_g_.m.nextp)
        _g_.m.nextp = nil
}

func mspinning() {
        getg().m.spinning = true
}

// Schedules some M to run the p (creates an M if necessary).
// If p==nil, tries to get an idle P, if no idle P's does nothing.
func startm(_p_ *p, spinning bool) {
        lock(&sched.lock)
        if _p_ == nil {
                _p_ = pidleget()
                if _p_ == nil {
                        unlock(&sched.lock)
                        if spinning {
                                xadd(&sched.nmspinning, -1)
                        }
                        return
                }
        }
        mp := mget()
        unlock(&sched.lock)
        if mp == nil {
                var fn func()
                if spinning {
                        fn = mspinning
                }
                _newm(fn, _p_)
                return
        }
        if mp.spinning {
                gothrow("startm: m is spinning")
        }
        if mp.nextp != nil {
                gothrow("startm: m has p")
        }
        mp.spinning = spinning
        mp.nextp = _p_
        notewakeup(&mp.park)
}

// Hands off P from syscall or locked M.
func handoffp(_p_ *p) {
        // if it has local work, start it straight away
        if _p_.runqhead != _p_.runqtail || sched.runqsize != 0 {
                startm(_p_, false)
                return
        }
        // no local work, check that there are no spinning/idle M's,
        // otherwise our help is not required
        if atomicload(&sched.nmspinning)+atomicload(&sched.npidle) == 0 && cas(&sched.nmspinning, 0, 1) { // TODO: fast atomic
                startm(_p_, true)
                return
        }
        lock(&sched.lock)
        if sched.gcwaiting != 0 {
                _p_.status = _Pgcstop
                sched.stopwait--
                if sched.stopwait == 0 {
                        notewakeup(&sched.stopnote)
                }
                unlock(&sched.lock)
                return
        }
        if sched.runqsize != 0 {
                unlock(&sched.lock)
                startm(_p_, false)
                return
        }
        // If this is the last running P and nobody is polling network,
        // need to wakeup another M to poll network.
        if sched.npidle == uint32(gomaxprocs-1) && atomicload64(&sched.lastpoll) != 0 {
                unlock(&sched.lock)
                startm(_p_, false)
                return
        }
        pidleput(_p_)
        unlock(&sched.lock)
}

// Tries to add one more P to execute G's.
// Called when a G is made runnable (newproc, ready).
func wakep() {
        // be conservative about spinning threads
        if !cas(&sched.nmspinning, 0, 1) {
                return
        }
        startm(nil, true)
}

// Stops execution of the current m that is locked to a g until the g is runnable again.
// Returns with acquired P.
func stoplockedm() {
        _g_ := getg()

        if _g_.m.lockedg == nil || _g_.m.lockedg.lockedm != _g_.m {
                gothrow("stoplockedm: inconsistent locking")
        }
        if _g_.m.p != nil {
                // Schedule another M to run this p.
                _p_ := releasep()
                handoffp(_p_)
        }
        incidlelocked(1)
        // Wait until another thread schedules lockedg again.
        notesleep(&_g_.m.park)
        noteclear(&_g_.m.park)
        status := readgstatus(_g_.m.lockedg)
        if status&^_Gscan != _Grunnable {
                print("runtime:stoplockedm: g is not Grunnable or Gscanrunnable\n")
                dumpgstatus(_g_)
                gothrow("stoplockedm: not runnable")
        }
        acquirep(_g_.m.nextp)
        _g_.m.nextp = nil
}

// Schedules the locked m to run the locked gp.
func startlockedm(gp *g) {
        _g_ := getg()

        mp := gp.lockedm
        if mp == _g_.m {
                gothrow("startlockedm: locked to me")
        }
        if mp.nextp != nil {
                gothrow("startlockedm: m has p")
        }
        // directly handoff current P to the locked m
        incidlelocked(-1)
        _p_ := releasep()
        mp.nextp = _p_
        notewakeup(&mp.park)
        stopm()
}

// Stops the current m for stoptheworld.
// Returns when the world is restarted.
func gcstopm() {
        _g_ := getg()

        if sched.gcwaiting == 0 {
                gothrow("gcstopm: not waiting for gc")
        }
        if _g_.m.spinning {
                _g_.m.spinning = false
                xadd(&sched.nmspinning, -1)
        }
        _p_ := releasep()
        lock(&sched.lock)
        _p_.status = _Pgcstop
        sched.stopwait--
        if sched.stopwait == 0 {
                notewakeup(&sched.stopnote)
        }
        unlock(&sched.lock)
        stopm()
}

// Schedules gp to run on the current M.
// Never returns.
func execute(gp *g) {
        _g_ := getg()

        casgstatus(gp, _Grunnable, _Grunning)
        gp.waitsince = 0
        gp.preempt = false
        gp.stackguard0 = gp.stack.lo + _StackGuard
        _g_.m.p.schedtick++
        _g_.m.curg = gp
        gp.m = _g_.m

        // Check whether the profiler needs to be turned on or off.
        hz := sched.profilehz
        if _g_.m.profilehz != hz {
                resetcpuprofiler(hz)
        }

        gogo(&gp.sched)
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from global queue, poll network.
func findrunnable() *g {
        _g_ := getg()

top:
        if sched.gcwaiting != 0 {
                gcstopm()
                goto top
        }
        if fingwait && fingwake {
                if gp := wakefing(); gp != nil {
                        ready(gp)
                }
        }

        // local runq
        if gp := runqget(_g_.m.p); gp != nil {
                return gp
        }

        // global runq
        if sched.runqsize != 0 {
                lock(&sched.lock)
                gp := globrunqget(_g_.m.p, 0)
                unlock(&sched.lock)
                if gp != nil {
                        return gp
                }
        }

        // poll network - returns list of goroutines
        if gp := netpoll(false); gp != nil { // non-blocking
                injectglist(gp.schedlink)
                casgstatus(gp, _Gwaiting, _Grunnable)
                return gp
        }

        // If number of spinning M's >= number of busy P's, block.
        // This is necessary to prevent excessive CPU consumption
        // when GOMAXPROCS>>1 but the program parallelism is low.
        if !_g_.m.spinning && 2*atomicload(&sched.nmspinning) >= uint32(gomaxprocs)-atomicload(&sched.npidle) { // TODO: fast atomic
                goto stop
        }
        if !_g_.m.spinning {
                _g_.m.spinning = true
                xadd(&sched.nmspinning, 1)
        }
        // random steal from other P's
        for i := 0; i < int(2*gomaxprocs); i++ {
                if sched.gcwaiting != 0 {
                        goto top
                }
                _p_ := allp[fastrand1()%uint32(gomaxprocs)]
                var gp *g
                if _p_ == _g_.m.p {
                        gp = runqget(_p_)
                } else {
                        gp = runqsteal(_g_.m.p, _p_)
                }
                if gp != nil {
                        return gp
                }
        }
stop:

        // return P and block
        lock(&sched.lock)
        if sched.gcwaiting != 0 {
                unlock(&sched.lock)
                goto top
        }
        if sched.runqsize != 0 {
                gp := globrunqget(_g_.m.p, 0)
                unlock(&sched.lock)
                return gp
        }
        _p_ := releasep()
        pidleput(_p_)
        unlock(&sched.lock)
        if _g_.m.spinning {
                _g_.m.spinning = false
                xadd(&sched.nmspinning, -1)
        }

        // check all runqueues once again
        for i := 0; i < int(gomaxprocs); i++ {
                _p_ := allp[i]
                if _p_ != nil && _p_.runqhead != _p_.runqtail {
                        lock(&sched.lock)
                        _p_ = pidleget()
                        unlock(&sched.lock)
                        if _p_ != nil {
                                acquirep(_p_)
                                goto top
                        }
                        break
                }
        }

        // poll network
        if xchg64(&sched.lastpoll, 0) != 0 {
                if _g_.m.p != nil {
                        gothrow("findrunnable: netpoll with p")
                }
                if _g_.m.spinning {
                        gothrow("findrunnable: netpoll with spinning")
                }
                gp := netpoll(true) // block until new work is available
                atomicstore64(&sched.lastpoll, uint64(nanotime()))
                if gp != nil {
                        lock(&sched.lock)
                        _p_ = pidleget()
                        unlock(&sched.lock)
                        if _p_ != nil {
                                acquirep(_p_)
                                injectglist(gp.schedlink)
                                casgstatus(gp, _Gwaiting, _Grunnable)
                                return gp
                        }
                        injectglist(gp)
                }
        }
        stopm()
        goto top
}

func resetspinning() {
        _g_ := getg()

        var nmspinning uint32
        if _g_.m.spinning {
                _g_.m.spinning = false
                nmspinning = xadd(&sched.nmspinning, -1)
                if nmspinning < 0 {
                        gothrow("findrunnable: negative nmspinning")
                }
        } else {
                nmspinning = atomicload(&sched.nmspinning)
        }

        // M wakeup policy is deliberately somewhat conservative (see nmspinning handling),
        // so see if we need to wakeup another P here.
        if nmspinning == 0 && atomicload(&sched.npidle) > 0 {
                wakep()
        }
}

// Injects the list of runnable G's into the scheduler.
// Can run concurrently with GC.
func injectglist(glist *g) {
        if glist == nil {
                return
        }
        lock(&sched.lock)
        var n int
        for n = 0; glist != nil; n++ {
                gp := glist
                glist = gp.schedlink
                casgstatus(gp, _Gwaiting, _Grunnable)
                globrunqput(gp)
        }
        unlock(&sched.lock)
        for ; n != 0 && sched.npidle != 0; n-- {
                startm(nil, false)
        }
}

// One round of scheduler: find a runnable goroutine and execute it.
// Never returns.
func schedule() {
        _g_ := getg()

        if _g_.m.locks != 0 {
                gothrow("schedule: holding locks")
        }

        if _g_.m.lockedg != nil {
                stoplockedm()
                execute(_g_.m.lockedg) // Never returns.
        }

top:
        if sched.gcwaiting != 0 {
                gcstopm()
                goto top
        }

        var gp *g
        // Check the global runnable queue once in a while to ensure fairness.
        // Otherwise two goroutines can completely occupy the local runqueue
        // by constantly respawning each other.
        tick := _g_.m.p.schedtick
        // This is a fancy way to say tick%61==0,
        // it uses 2 MUL instructions instead of a single DIV and so is faster on modern processors.
        if uint64(tick)-((uint64(tick)*0x4325c53f)>>36)*61 == 0 && sched.runqsize > 0 {
                lock(&sched.lock)
                gp = globrunqget(_g_.m.p, 1)
                unlock(&sched.lock)
                if gp != nil {
                        resetspinning()
                }
        }
        if gp == nil {
                gp = runqget(_g_.m.p)
                if gp != nil && _g_.m.spinning {
                        gothrow("schedule: spinning with local work")
                }
        }
        if gp == nil {
                gp = findrunnable() // blocks until work is available
                resetspinning()
        }

        if gp.lockedm != nil {
                // Hands off own p to the locked m,
                // then blocks waiting for a new p.
                startlockedm(gp)
                goto top
        }

        execute(gp)
}

// dropg removes the association between m and the current goroutine m->curg (gp for short).
// Typically a caller sets gp's status away from Grunning and then
// immediately calls dropg to finish the job. The caller is also responsible
// for arranging that gp will be restarted using ready at an
// appropriate time. After calling dropg and arranging for gp to be
// readied later, the caller can do other work but eventually should
// call schedule to restart the scheduling of goroutines on this m.
func dropg() {
        _g_ := getg()

        if _g_.m.lockedg == nil {
                _g_.m.curg.m = nil
                _g_.m.curg = nil
        }
}

// Puts the current goroutine into a waiting state and calls unlockf.
// If unlockf returns false, the goroutine is resumed.
func park(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason string) {
        _g_ := getg()

        _g_.m.waitlock = lock
        _g_.m.waitunlockf = *(*unsafe.Pointer)(unsafe.Pointer(&unlockf))
        _g_.waitreason = reason
        mcall(park_m)
}

func parkunlock_c(gp *g, lock unsafe.Pointer) bool {
        unlock((*mutex)(lock))
        return true
}

// Puts the current goroutine into a waiting state and unlocks the lock.
// The goroutine can be made runnable again by calling ready(gp).
func parkunlock(lock *mutex, reason string) {
        park(parkunlock_c, unsafe.Pointer(lock), reason)
}

// park continuation on g0.
func park_m(gp *g) {
        _g_ := getg()

        casgstatus(gp, _Grunning, _Gwaiting)
        dropg()

        if _g_.m.waitunlockf != nil {
                fn := *(*func(*g, unsafe.Pointer) bool)(unsafe.Pointer(&_g_.m.waitunlockf))
                ok := fn(gp, _g_.m.waitlock)
                _g_.m.waitunlockf = nil
                _g_.m.waitlock = nil
                if !ok {
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        execute(gp) // Schedule it back, never returns.
                }
        }
        schedule()
}

// Gosched continuation on g0.
func gosched_m(gp *g) {
        status := readgstatus(gp)
        if status&^_Gscan != _Grunning {
                dumpgstatus(gp)
                gothrow("bad g status")
        }
        casgstatus(gp, _Grunning, _Grunnable)
        dropg()
        lock(&sched.lock)
        globrunqput(gp)
        unlock(&sched.lock)

        schedule()
}

// Finishes execution of the current goroutine.
// Must be NOSPLIT because it is called from Go. (TODO - probably not anymore)
//go:nosplit
func goexit1() {
        if raceenabled {
                racegoend()
        }
        mcall(goexit0)
}

// goexit continuation on g0.
func goexit0(gp *g) {
        _g_ := getg()

        casgstatus(gp, _Grunning, _Gdead)
        gp.m = nil
        gp.lockedm = nil
        _g_.m.lockedg = nil
        gp.paniconfault = false
        gp._defer = nil // should be true already but just in case.
        gp._panic = nil // non-nil for Goexit during panic. points at stack-allocated data.
        gp.writebuf = nil
        gp.waitreason = ""
        gp.param = nil

        dropg()

        if _g_.m.locked&^_LockExternal != 0 {
                print("invalid m->locked = ", _g_.m.locked, "\n")
                gothrow("internal lockOSThread error")
        }
        _g_.m.locked = 0
        gfput(_g_.m.p, gp)
        schedule()
}

//go:nosplit
func save(pc, sp uintptr) {
        _g_ := getg()

        _g_.sched.pc = pc
        _g_.sched.sp = sp
        _g_.sched.lr = 0
        _g_.sched.ret = 0
        _g_.sched.ctxt = nil
        // write as uintptr to avoid write barrier, which will smash _g_.sched.
        *(*uintptr)(unsafe.Pointer(&_g_.sched.g)) = uintptr(unsafe.Pointer(_g_))
}

// The goroutine g is about to enter a system call.
// Record that it's not using the cpu anymore.
// This is called only from the go syscall library and cgocall,
// not from the low-level system calls used by the
//
// Entersyscall cannot split the stack: the gosave must
// make g->sched refer to the caller's stack segment, because
// entersyscall is going to return immediately after.
//
// Nothing entersyscall calls can split the stack either.
// We cannot safely move the stack during an active call to syscall,
// because we do not know which of the uintptr arguments are
// really pointers (back into the stack).
// In practice, this means that we make the fast path run through
// entersyscall doing no-split things, and the slow path has to use systemstack
// to run bigger things on the system stack.
//
// reentersyscall is the entry point used by cgo callbacks, where explicitly
// saved SP and PC are restored. This is needed when exitsyscall will be called
// from a function further up in the call stack than the parent, as g->syscallsp
// must always point to a valid stack frame. entersyscall below is the normal
// entry point for syscalls, which obtains the SP and PC from the caller.
//go:nosplit
func reentersyscall(pc, sp uintptr) {
        _g_ := getg()

        // Disable preemption because during this function g is in Gsyscall status,
        // but can have inconsistent g->sched, do not let GC observe it.
        _g_.m.locks++

        // Entersyscall must not call any function that might split/grow the stack.
        // (See details in comment above.)
        // Catch calls that might, by replacing the stack guard with something that
        // will trip any stack check and leaving a flag to tell newstack to die.
        _g_.stackguard0 = stackPreempt
        _g_.throwsplit = true

        // Leave SP around for GC and traceback.
        save(pc, sp)
        _g_.syscallsp = sp
        _g_.syscallpc = pc
        casgstatus(_g_, _Grunning, _Gsyscall)
        if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
                systemstack(func() {
                        print("entersyscall inconsistent ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
                        gothrow("entersyscall")
                })
        }

        if atomicload(&sched.sysmonwait) != 0 { // TODO: fast atomic
                systemstack(entersyscall_sysmon)
                save(pc, sp)
        }

        _g_.m.mcache = nil
        _g_.m.p.m = nil
        atomicstore(&_g_.m.p.status, _Psyscall)
        if sched.gcwaiting != 0 {
                systemstack(entersyscall_gcwait)
                save(pc, sp)
        }

        // Goroutines must not split stacks in Gsyscall status (it would corrupt g->sched).
        // We set _StackGuard to StackPreempt so that first split stack check calls morestack.
        // Morestack detects this case and throws.
        _g_.stackguard0 = stackPreempt
        _g_.m.locks--
}

// Standard syscall entry used by the go syscall library and normal cgo calls.
//go:nosplit
func entersyscall(dummy int32) {
        reentersyscall(getcallerpc(unsafe.Pointer(&dummy)), getcallersp(unsafe.Pointer(&dummy)))
}

func entersyscall_sysmon() {
        lock(&sched.lock)
        if atomicload(&sched.sysmonwait) != 0 {
                atomicstore(&sched.sysmonwait, 0)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
}

func entersyscall_gcwait() {
        _g_ := getg()

        lock(&sched.lock)
        if sched.stopwait > 0 && cas(&_g_.m.p.status, _Psyscall, _Pgcstop) {
                if sched.stopwait--; sched.stopwait == 0 {
                        notewakeup(&sched.stopnote)
                }
        }
        unlock(&sched.lock)
}

// The same as entersyscall(), but with a hint that the syscall is blocking.
//go:nosplit
func entersyscallblock(dummy int32) {
        _g_ := getg()

        _g_.m.locks++ // see comment in entersyscall
        _g_.throwsplit = true
        _g_.stackguard0 = stackPreempt // see comment in entersyscall

        // Leave SP around for GC and traceback.
        pc := getcallerpc(unsafe.Pointer(&dummy))
        sp := getcallersp(unsafe.Pointer(&dummy))
        save(pc, sp)
        _g_.syscallsp = _g_.sched.sp
        _g_.syscallpc = _g_.sched.pc
        if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
                sp1 := sp
                sp2 := _g_.sched.sp
                sp3 := _g_.syscallsp
                systemstack(func() {
                        print("entersyscallblock inconsistent ", hex(sp1), " ", hex(sp2), " ", hex(sp3), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
                        gothrow("entersyscallblock")
                })
        }
        casgstatus(_g_, _Grunning, _Gsyscall)
        if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
                systemstack(func() {
                        print("entersyscallblock inconsistent ", hex(sp), " ", hex(_g_.sched.sp), " ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
                        gothrow("entersyscallblock")
                })
        }

        systemstack(entersyscallblock_handoff)

        // Resave for traceback during blocked call.
        save(getcallerpc(unsafe.Pointer(&dummy)), getcallersp(unsafe.Pointer(&dummy)))

        _g_.m.locks--
}

func entersyscallblock_handoff() {
        handoffp(releasep())
}

// The goroutine g exited its system call.
// Arrange for it to run on a cpu again.
// This is called only from the go syscall library, not
// from the low-level system calls used by the
//go:nosplit
func exitsyscall(dummy int32) {
        _g_ := getg()

        _g_.m.locks++ // see comment in entersyscall
        if getcallersp(unsafe.Pointer(&dummy)) > _g_.syscallsp {
                gothrow("exitsyscall: syscall frame is no longer valid")
        }

        _g_.waitsince = 0
        if exitsyscallfast() {
                if _g_.m.mcache == nil {
                        gothrow("lost mcache")
                }
                // There's a cpu for us, so we can run.
                _g_.m.p.syscalltick++
                // We need to cas the status and scan before resuming...
                casgstatus(_g_, _Gsyscall, _Grunning)

                // Garbage collector isn't running (since we are),
                // so okay to clear syscallsp.
                _g_.syscallsp = 0
                _g_.m.locks--
                if _g_.preempt {
                        // restore the preemption request in case we've cleared it in newstack
                        _g_.stackguard0 = stackPreempt
                } else {
                        // otherwise restore the real _StackGuard, we've spoiled it in entersyscall/entersyscallblock
                        _g_.stackguard0 = _g_.stack.lo + _StackGuard
                }
                _g_.throwsplit = false
                return
        }

        _g_.m.locks--

        // Call the scheduler.
        mcall(exitsyscall0)

        if _g_.m.mcache == nil {
                gothrow("lost mcache")
        }

        // Scheduler returned, so we're allowed to run now.
        // Delete the syscallsp information that we left for
        // the garbage collector during the system call.
        // Must wait until now because until gosched returns
        // we don't know for sure that the garbage collector
        // is not running.
        _g_.syscallsp = 0
        _g_.m.p.syscalltick++
        _g_.throwsplit = false
}

//go:nosplit
func exitsyscallfast() bool {
        _g_ := getg()

        // Freezetheworld sets stopwait but does not retake P's.
        if sched.stopwait != 0 {
                _g_.m.mcache = nil
                _g_.m.p = nil
                return false
        }

        // Try to re-acquire the last P.
        if _g_.m.p != nil && _g_.m.p.status == _Psyscall && cas(&_g_.m.p.status, _Psyscall, _Prunning) {
                // There's a cpu for us, so we can run.
                _g_.m.mcache = _g_.m.p.mcache
                _g_.m.p.m = _g_.m
                return true
        }

        // Try to get any other idle P.
        _g_.m.mcache = nil
        _g_.m.p = nil
        if sched.pidle != nil {
                var ok bool
                systemstack(func() {
                        ok = exitsyscallfast_pidle()
                })
                if ok {
                        return true
                }
        }
        return false
}

func exitsyscallfast_pidle() bool {
        lock(&sched.lock)
        _p_ := pidleget()
        if _p_ != nil && atomicload(&sched.sysmonwait) != 0 {
                atomicstore(&sched.sysmonwait, 0)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
        if _p_ != nil {
                acquirep(_p_)
                return true
        }
        return false
}

// exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
func exitsyscall0(gp *g) {
        _g_ := getg()

        casgstatus(gp, _Gsyscall, _Grunnable)
        dropg()
        lock(&sched.lock)
        _p_ := pidleget()
        if _p_ == nil {
                globrunqput(gp)
        } else if atomicload(&sched.sysmonwait) != 0 {
                atomicstore(&sched.sysmonwait, 0)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
        if _p_ != nil {
                acquirep(_p_)
                execute(gp) // Never returns.
        }
        if _g_.m.lockedg != nil {
                // Wait until another thread schedules gp and so m again.
                stoplockedm()
                execute(gp) // Never returns.
        }
        stopm()
        schedule() // Never returns.
}

func beforefork() {
        gp := getg().m.curg

        // Fork can hang if preempted with signals frequently enough (see issue 5517).
        // Ensure that we stay on the same M where we disable profiling.
        gp.m.locks++
        if gp.m.profilehz != 0 {
                resetcpuprofiler(0)
        }

        // This function is called before fork in syscall package.
        // Code between fork and exec must not allocate memory nor even try to grow stack.
        // Here we spoil g->_StackGuard to reliably detect any attempts to grow stack.
        // runtime_AfterFork will undo this in parent process, but not in child.
        gp.stackguard0 = stackFork
}

// Called from syscall package before fork.
//go:nosplit
func syscall_BeforeFork() {
        systemstack(beforefork)
}

func afterfork() {
        gp := getg().m.curg

        // See the comment in beforefork.
        gp.stackguard0 = gp.stack.lo + _StackGuard

        hz := sched.profilehz
        if hz != 0 {
                resetcpuprofiler(hz)
        }
        gp.m.locks--
}

// Called from syscall package after fork in parent.
//go:nosplit
func syscall_AfterFork() {
        systemstack(afterfork)
}

// Allocate a new g, with a stack big enough for stacksize bytes.
func malg(stacksize int32) *g {
        newg := allocg()
        if stacksize >= 0 {
                stacksize = round2(_StackSystem + stacksize)
                systemstack(func() {
                        newg.stack = stackalloc(uint32(stacksize))
                })
                newg.stackguard0 = newg.stack.lo + _StackGuard
                newg.stackguard1 = ^uintptr(0)
        }
        return newg
}

// Create a new g running fn with siz bytes of arguments.
// Put it on the queue of g's waiting to run.
// The compiler turns a go statement into a call to this.
// Cannot split the stack because it assumes that the arguments
// are available sequentially after &fn; they would not be
// copied if a stack split occurred.
//go:nosplit
func newproc(siz int32, fn *funcval) {
        argp := add(unsafe.Pointer(&fn), ptrSize)
        if hasLinkRegister {
                argp = add(argp, ptrSize) // skip caller's saved LR
        }

        pc := getcallerpc(unsafe.Pointer(&siz))
        systemstack(func() {
                newproc1(fn, (*uint8)(argp), siz, 0, pc)
        })
}

// Create a new g running fn with narg bytes of arguments starting
// at argp and returning nret bytes of results.  callerpc is the
// address of the go statement that created this.  The new g is put
// on the queue of g's waiting to run.
func newproc1(fn *funcval, argp *uint8, narg int32, nret int32, callerpc uintptr) *g {
        _g_ := getg()

        if fn == nil {
                _g_.m.throwing = -1 // do not dump full stacks
                gothrow("go of nil func value")
        }
        _g_.m.locks++ // disable preemption because it can be holding p in a local var
        siz := narg + nret
        siz = (siz + 7) &^ 7

        // We could allocate a larger initial stack if necessary.
        // Not worth it: this is almost always an error.
        // 4*sizeof(uintreg): extra space added below
        // sizeof(uintreg): caller's LR (arm) or return address (x86, in gostartcall).
        if siz >= _StackMin-4*regSize-regSize {
                gothrow("newproc: function arguments too large for new goroutine")
        }

        _p_ := _g_.m.p
        newg := gfget(_p_)
        if newg == nil {
                newg = malg(_StackMin)
                casgstatus(newg, _Gidle, _Gdead)
                allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
        }
        if newg.stack.hi == 0 {
                gothrow("newproc1: newg missing stack")
        }

        if readgstatus(newg) != _Gdead {
                gothrow("newproc1: new g is not Gdead")
        }

        sp := newg.stack.hi
        sp -= 4 * regSize // extra space in case of reads slightly beyond frame
        sp -= uintptr(siz)
        memmove(unsafe.Pointer(sp), unsafe.Pointer(argp), uintptr(narg))
        if hasLinkRegister {
                // caller's LR
                sp -= ptrSize
                *(*unsafe.Pointer)(unsafe.Pointer(sp)) = nil
        }

        memclr(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))
        newg.sched.sp = sp
        newg.sched.pc = funcPC(goexit) + _PCQuantum // +PCQuantum so that previous instruction is in same function
        newg.sched.g = newg
        gostartcallfn(&newg.sched, fn)
        newg.gopc = callerpc
        casgstatus(newg, _Gdead, _Grunnable)

        if _p_.goidcache == _p_.goidcacheend {
                // Sched.goidgen is the last allocated id,
                // this batch must be [sched.goidgen+1, sched.goidgen+GoidCacheBatch].
                // At startup sched.goidgen=0, so main goroutine receives goid=1.
                _p_.goidcache = xadd64(&sched.goidgen, _GoidCacheBatch)
                _p_.goidcache -= _GoidCacheBatch - 1
                _p_.goidcacheend = _p_.goidcache + _GoidCacheBatch
        }
        newg.goid = int64(_p_.goidcache)
        _p_.goidcache++
        if raceenabled {
                newg.racectx = racegostart(callerpc)
        }
        runqput(_p_, newg)

        if atomicload(&sched.npidle) != 0 && atomicload(&sched.nmspinning) == 0 && unsafe.Pointer(fn.fn) != unsafe.Pointer(funcPC(main)) { // TODO: fast atomic
                wakep()
        }
        _g_.m.locks--
        if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
                _g_.stackguard0 = stackPreempt
        }
        return newg
}

// Put on gfree list.
// If local list is too long, transfer a batch to the global list.
func gfput(_p_ *p, gp *g) {
        if readgstatus(gp) != _Gdead {
                gothrow("gfput: bad status (not Gdead)")
        }

        stksize := gp.stack.hi - gp.stack.lo

        if stksize != _FixedStack {
                // non-standard stack size - free it.
                stackfree(gp.stack)
                gp.stack.lo = 0
                gp.stack.hi = 0
                gp.stackguard0 = 0
        }

        gp.schedlink = _p_.gfree
        _p_.gfree = gp
        _p_.gfreecnt++
        if _p_.gfreecnt >= 64 {
                lock(&sched.gflock)
                for _p_.gfreecnt >= 32 {
                        _p_.gfreecnt--
                        gp = _p_.gfree
                        _p_.gfree = gp.schedlink
                        gp.schedlink = sched.gfree
                        sched.gfree = gp
                        sched.ngfree++
                }
                unlock(&sched.gflock)
        }
}

// Get from gfree list.
// If local list is empty, grab a batch from global list.
func gfget(_p_ *p) *g {
retry:
        gp := _p_.gfree
        if gp == nil && sched.gfree != nil {
                lock(&sched.gflock)
                for _p_.gfreecnt < 32 && sched.gfree != nil {
                        _p_.gfreecnt++
                        gp = sched.gfree
                        sched.gfree = gp.schedlink
                        sched.ngfree--
                        gp.schedlink = _p_.gfree
                        _p_.gfree = gp
                }
                unlock(&sched.gflock)
                goto retry
        }
        if gp != nil {
                _p_.gfree = gp.schedlink
                _p_.gfreecnt--
                if gp.stack.lo == 0 {
                        // Stack was deallocated in gfput.  Allocate a new one.
                        systemstack(func() {
                                gp.stack = stackalloc(_FixedStack)
                        })
                        gp.stackguard0 = gp.stack.lo + _StackGuard
                } else {
                        if raceenabled {
                                racemalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
                        }
                }
        }
        return gp
}

// Purge all cached G's from gfree list to the global list.
func gfpurge(_p_ *p) {
        lock(&sched.gflock)
        for _p_.gfreecnt != 0 {
                _p_.gfreecnt--
                gp := _p_.gfree
                _p_.gfree = gp.schedlink
                gp.schedlink = sched.gfree
                sched.gfree = gp
                sched.ngfree++
        }
        unlock(&sched.gflock)
}

// Breakpoint executes a breakpoint trap.
func Breakpoint() {
        breakpoint()
}

// dolockOSThread is called by LockOSThread and lockOSThread below
// after they modify m.locked. Do not allow preemption during this call,
// or else the m might be different in this function than in the caller.
//go:nosplit
func dolockOSThread() {
        _g_ := getg()
        _g_.m.lockedg = _g_
        _g_.lockedm = _g_.m
}

//go:nosplit

// LockOSThread wires the calling goroutine to its current operating system thread.
// Until the calling goroutine exits or calls UnlockOSThread, it will always
// execute in that thread, and no other goroutine can.
func LockOSThread() {
        getg().m.locked |= _LockExternal
        dolockOSThread()
}

//go:nosplit
func lockOSThread() {
        getg().m.locked += _LockInternal
        dolockOSThread()
}

// dounlockOSThread is called by UnlockOSThread and unlockOSThread below
// after they update m->locked. Do not allow preemption during this call,
// or else the m might be in different in this function than in the caller.
//go:nosplit
func dounlockOSThread() {
        _g_ := getg()
        if _g_.m.locked != 0 {
                return
        }
        _g_.m.lockedg = nil
        _g_.lockedm = nil
}

//go:nosplit

// UnlockOSThread unwires the calling goroutine from its fixed operating system thread.
// If the calling goroutine has not called LockOSThread, UnlockOSThread is a no-op.
func UnlockOSThread() {
        getg().m.locked &^= _LockExternal
        dounlockOSThread()
}

//go:nosplit
func unlockOSThread() {
        _g_ := getg()
        if _g_.m.locked < _LockInternal {
                systemstack(badunlockosthread)
        }
        _g_.m.locked -= _LockInternal
        dounlockOSThread()
}

func badunlockosthread() {
        gothrow("runtime: internal error: misuse of lockOSThread/unlockOSThread")
}

func gcount() int32 {
        n := int32(allglen) - sched.ngfree
        for i := 0; ; i++ {
                _p_ := allp[i]
                if _p_ == nil {
                        break
                }
                n -= _p_.gfreecnt
        }

        // All these variables can be changed concurrently, so the result can be inconsistent.
        // But at least the current goroutine is running.
        if n < 1 {
                n = 1
        }
        return n
}

func mcount() int32 {
        return sched.mcount
}

var prof struct {
        lock uint32
        hz   int32
}

func _System()       { _System() }
func _ExternalCode() { _ExternalCode() }
func _GC()           { _GC() }

var etext struct{}

// Called if we receive a SIGPROF signal.
func sigprof(pc *uint8, sp *uint8, lr *uint8, gp *g, mp *m) {
        var n int32
        var traceback bool
        var stk [100]uintptr

        if prof.hz == 0 {
                return
        }

        // Profiling runs concurrently with GC, so it must not allocate.
        mp.mallocing++

        // Define that a "user g" is a user-created goroutine, and a "system g"
        // is one that is m->g0 or m->gsignal. We've only made sure that we
        // can unwind user g's, so exclude the system g's.
        //
        // It is not quite as easy as testing gp == m->curg (the current user g)
        // because we might be interrupted for profiling halfway through a
        // goroutine switch. The switch involves updating three (or four) values:
        // g, PC, SP, and (on arm) LR. The PC must be the last to be updated,
        // because once it gets updated the new g is running.
        //
        // When switching from a user g to a system g, LR is not considered live,
        // so the update only affects g, SP, and PC. Since PC must be last, there
        // the possible partial transitions in ordinary execution are (1) g alone is updated,
        // (2) both g and SP are updated, and (3) SP alone is updated.
        // If g is updated, we'll see a system g and not look closer.
        // If SP alone is updated, we can detect the partial transition by checking
        // whether the SP is within g's stack bounds. (We could also require that SP
        // be changed only after g, but the stack bounds check is needed by other
        // cases, so there is no need to impose an additional requirement.)
        //
        // There is one exceptional transition to a system g, not in ordinary execution.
        // When a signal arrives, the operating system starts the signal handler running
        // with an updated PC and SP. The g is updated last, at the beginning of the
        // handler. There are two reasons this is okay. First, until g is updated the
        // g and SP do not match, so the stack bounds check detects the partial transition.
        // Second, signal handlers currently run with signals disabled, so a profiling
        // signal cannot arrive during the handler.
        //
        // When switching from a system g to a user g, there are three possibilities.
        //
        // First, it may be that the g switch has no PC update, because the SP
        // either corresponds to a user g throughout (as in asmcgocall)
        // or because it has been arranged to look like a user g frame
        // (as in cgocallback_gofunc). In this case, since the entire
        // transition is a g+SP update, a partial transition updating just one of
        // those will be detected by the stack bounds check.
        //
        // Second, when returning from a signal handler, the PC and SP updates
        // are performed by the operating system in an atomic update, so the g
        // update must be done before them. The stack bounds check detects
        // the partial transition here, and (again) signal handlers run with signals
        // disabled, so a profiling signal cannot arrive then anyway.
        //
        // Third, the common case: it may be that the switch updates g, SP, and PC
        // separately, as in gogo.
        //
        // Because gogo is the only instance, we check whether the PC lies
        // within that function, and if so, not ask for a traceback. This approach
        // requires knowing the size of the gogo function, which we
        // record in arch_*.h and check in runtime_test.go.
        //
        // There is another apparently viable approach, recorded here in case
        // the "PC within gogo" check turns out not to be usable.
        // It would be possible to delay the update of either g or SP until immediately
        // before the PC update instruction. Then, because of the stack bounds check,
        // the only problematic interrupt point is just before that PC update instruction,
        // and the sigprof handler can detect that instruction and simulate stepping past
        // it in order to reach a consistent state. On ARM, the update of g must be made
        // in two places (in R10 and also in a TLS slot), so the delayed update would
        // need to be the SP update. The sigprof handler must read the instruction at
        // the current PC and if it was the known instruction (for example, JMP BX or
        // MOV R2, PC), use that other register in place of the PC value.
        // The biggest drawback to this solution is that it requires that we can tell
        // whether it's safe to read from the memory pointed at by PC.
        // In a correct program, we can test PC == nil and otherwise read,
        // but if a profiling signal happens at the instant that a program executes
        // a bad jump (before the program manages to handle the resulting fault)
        // the profiling handler could fault trying to read nonexistent memory.
        //
        // To recap, there are no constraints on the assembly being used for the
        // transition. We simply require that g and SP match and that the PC is not
        // in gogo.
        traceback = true
        usp := uintptr(unsafe.Pointer(sp))
        gogo := funcPC(gogo)
        if gp == nil || gp != mp.curg ||
                usp < gp.stack.lo || gp.stack.hi < usp ||
                (gogo <= uintptr(unsafe.Pointer(pc)) && uintptr(unsafe.Pointer(pc)) < gogo+_RuntimeGogoBytes) {
                traceback = false
        }

        n = 0
        if traceback {
                n = int32(gentraceback(uintptr(unsafe.Pointer(pc)), uintptr(unsafe.Pointer(sp)), uintptr(unsafe.Pointer(lr)), gp, 0, &stk[0], len(stk), nil, nil, _TraceTrap))
        }
        if !traceback || n <= 0 {
                // Normal traceback is impossible or has failed.
                // See if it falls into several common cases.
                n = 0
                if mp.ncgo > 0 && mp.curg != nil && mp.curg.syscallpc != 0 && mp.curg.syscallsp != 0 {
                        // Cgo, we can't unwind and symbolize arbitrary C code,
                        // so instead collect Go stack that leads to the cgo call.
                        // This is especially important on windows, since all syscalls are cgo calls.
                        n = int32(gentraceback(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, 0, &stk[0], len(stk), nil, nil, 0))
                }
                if GOOS == "windows" && n == 0 && mp.libcallg != nil && mp.libcallpc != 0 && mp.libcallsp != 0 {
                        // Libcall, i.e. runtime syscall on windows.
                        // Collect Go stack that leads to the call.
                        n = int32(gentraceback(mp.libcallpc, mp.libcallsp, 0, mp.libcallg, 0, &stk[0], len(stk), nil, nil, 0))
                }
                if n == 0 {
                        // If all of the above has failed, account it against abstract "System" or "GC".
                        n = 2
                        // "ExternalCode" is better than "etext".
                        if uintptr(unsafe.Pointer(pc)) > uintptr(unsafe.Pointer(&etext)) {
                                pc = (*uint8)(unsafe.Pointer(uintptr(funcPC(_ExternalCode) + _PCQuantum)))
                        }
                        stk[0] = uintptr(unsafe.Pointer(pc))
                        if mp.gcing != 0 || mp.helpgc != 0 {
                                stk[1] = funcPC(_GC) + _PCQuantum
                        } else {
                                stk[1] = funcPC(_System) + _PCQuantum
                        }
                }
        }

        if prof.hz != 0 {
                // Simple cas-lock to coordinate with setcpuprofilerate.
                for !cas(&prof.lock, 0, 1) {
                        osyield()
                }
                if prof.hz != 0 {
                        cpuproftick(&stk[0], n)
                }
                atomicstore(&prof.lock, 0)
        }
        mp.mallocing--
}

// Arrange to call fn with a traceback hz times a second.
func setcpuprofilerate_m(hz int32) {
        // Force sane arguments.
        if hz < 0 {
                hz = 0
        }

        // Disable preemption, otherwise we can be rescheduled to another thread
        // that has profiling enabled.
        _g_ := getg()
        _g_.m.locks++

        // Stop profiler on this thread so that it is safe to lock prof.
        // if a profiling signal came in while we had prof locked,
        // it would deadlock.
        resetcpuprofiler(0)

        for !cas(&prof.lock, 0, 1) {
                osyield()
        }
        prof.hz = hz
        atomicstore(&prof.lock, 0)

        lock(&sched.lock)
        sched.profilehz = hz
        unlock(&sched.lock)

        if hz != 0 {
                resetcpuprofiler(hz)
        }

        _g_.m.locks--
}

// Change number of processors.  The world is stopped, sched is locked.
// gcworkbufs are not being modified by either the GC or
// the write barrier code.
func procresize(new int32) {
        old := gomaxprocs
        if old < 0 || old > _MaxGomaxprocs || new <= 0 || new > _MaxGomaxprocs {
                gothrow("procresize: invalid arg")
        }

        // initialize new P's
        for i := int32(0); i < new; i++ {
                p := allp[i]
                if p == nil {
                        p = newP()
                        p.id = i
                        p.status = _Pgcstop
                        atomicstorep(unsafe.Pointer(&allp[i]), unsafe.Pointer(p))
                }
                if p.mcache == nil {
                        if old == 0 && i == 0 {
                                if getg().m.mcache == nil {
                                        gothrow("missing mcache?")
                                }
                                p.mcache = getg().m.mcache // bootstrap
                        } else {
                                p.mcache = allocmcache()
                        }
                }
        }

        // redistribute runnable G's evenly
        // collect all runnable goroutines in global queue preserving FIFO order
        // FIFO order is required to ensure fairness even during frequent GCs
        // see http://golang.org/issue/7126
        empty := false
        for !empty {
                empty = true
                for i := int32(0); i < old; i++ {
                        p := allp[i]
                        if p.runqhead == p.runqtail {
                                continue
                        }
                        empty = false
                        // pop from tail of local queue
                        p.runqtail--
                        gp := p.runq[p.runqtail%uint32(len(p.runq))]
                        // push onto head of global queue
                        gp.schedlink = sched.runqhead
                        sched.runqhead = gp
                        if sched.runqtail == nil {
                                sched.runqtail = gp
                        }
                        sched.runqsize++
                }
        }

        // fill local queues with at most len(p.runq)/2 goroutines
        // start at 1 because current M already executes some G and will acquire allp[0] below,
        // so if we have a spare G we want to put it into allp[1].
        var _p_ p
        for i := int32(1); i < new*int32(len(_p_.runq))/2 && sched.runqsize > 0; i++ {
                gp := sched.runqhead
                sched.runqhead = gp.schedlink
                if sched.runqhead == nil {
                        sched.runqtail = nil
                }
                sched.runqsize--
                runqput(allp[i%new], gp)
        }

        // free unused P's
        for i := new; i < old; i++ {
                p := allp[i]
                freemcache(p.mcache)
                p.mcache = nil
                gfpurge(p)
                p.status = _Pdead
                // can't free P itself because it can be referenced by an M in syscall
        }

        _g_ := getg()
        if _g_.m.p != nil {
                _g_.m.p.m = nil
        }
        _g_.m.p = nil
        _g_.m.mcache = nil
        p := allp[0]
        p.m = nil
        p.status = _Pidle
        acquirep(p)
        for i := new - 1; i > 0; i-- {
                p := allp[i]
                p.status = _Pidle
                pidleput(p)
        }
        var int32p *int32 = &gomaxprocs // make compiler check that gomaxprocs is an int32
        atomicstore((*uint32)(unsafe.Pointer(int32p)), uint32(new))
}

// Associate p and the current m.
func acquirep(_p_ *p) {
        _g_ := getg()

        if _g_.m.p != nil || _g_.m.mcache != nil {
                gothrow("acquirep: already in go")
        }
        if _p_.m != nil || _p_.status != _Pidle {
                id := int32(0)
                if _p_.m != nil {
                        id = _p_.m.id
                }
                print("acquirep: p->m=", _p_.m, "(", id, ") p->status=", _p_.status, "\n")
                gothrow("acquirep: invalid p state")
        }
        _g_.m.mcache = _p_.mcache
        _g_.m.p = _p_
        _p_.m = _g_.m
        _p_.status = _Prunning
}

// Disassociate p and the current m.
func releasep() *p {
        _g_ := getg()

        if _g_.m.p == nil || _g_.m.mcache == nil {
                gothrow("releasep: invalid arg")
        }
        _p_ := _g_.m.p
        if _p_.m != _g_.m || _p_.mcache != _g_.m.mcache || _p_.status != _Prunning {
                print("releasep: m=", _g_.m, " m->p=", _g_.m.p, " p->m=", _p_.m, " m->mcache=", _g_.m.mcache, " p->mcache=", _p_.mcache, " p->status=", _p_.status, "\n")
                gothrow("releasep: invalid p state")
        }
        _g_.m.p = nil
        _g_.m.mcache = nil
        _p_.m = nil
        _p_.status = _Pidle
        return _p_
}

func incidlelocked(v int32) {
        lock(&sched.lock)
        sched.nmidlelocked += v
        if v > 0 {
                checkdead()
        }
        unlock(&sched.lock)
}

// Check for deadlock situation.
// The check is based on number of running M's, if 0 -> deadlock.
func checkdead() {
        // If we are dying because of a signal caught on an already idle thread,
        // freezetheworld will cause all running threads to block.
        // And runtime will essentially enter into deadlock state,
        // except that there is a thread that will call exit soon.
        if panicking > 0 {
                return
        }

        // -1 for sysmon
        run := sched.mcount - sched.nmidle - sched.nmidlelocked - 1
        if run > 0 {
                return
        }
        if run < 0 {
                print("runtime: checkdead: nmidle=", sched.nmidle, " nmidlelocked=", sched.nmidlelocked, " mcount=", sched.mcount, "\n")
                gothrow("checkdead: inconsistent counts")
        }

        grunning := 0
        lock(&allglock)
        for i := 0; i < len(allgs); i++ {
                gp := allgs[i]
                if gp.issystem {
                        continue
                }
                s := readgstatus(gp)
                switch s &^ _Gscan {
                case _Gwaiting:
                        grunning++
                case _Grunnable,
                        _Grunning,
                        _Gsyscall:
                        unlock(&allglock)
                        print("runtime: checkdead: find g ", gp.goid, " in status ", s, "\n")
                        gothrow("checkdead: runnable g")
                }
        }
        unlock(&allglock)
        if grunning == 0 { // possible if main goroutine calls runtime·Goexit()
                gothrow("no goroutines (main called runtime.Goexit) - deadlock!")
        }

        // Maybe jump time forward for playground.
        gp := timejump()
        if gp != nil {
                casgstatus(gp, _Gwaiting, _Grunnable)
                globrunqput(gp)
                _p_ := pidleget()
                if _p_ == nil {
                        gothrow("checkdead: no p for timer")
                }
                mp := mget()
                if mp == nil {
                        _newm(nil, _p_)
                } else {
                        mp.nextp = _p_
                        notewakeup(&mp.park)
                }
                return
        }

        getg().m.throwing = -1 // do not dump full stacks
        gothrow("all goroutines are asleep - deadlock!")
}

func sysmon() {
        // If we go two minutes without a garbage collection, force one to run.
        forcegcperiod := int64(2 * 60 * 1e9)

        // If a heap span goes unused for 5 minutes after a garbage collection,
        // we hand it back to the operating system.
        scavengelimit := int64(5 * 60 * 1e9)

        if debug.scavenge > 0 {
                // Scavenge-a-lot for testing.
                forcegcperiod = 10 * 1e6
                scavengelimit = 20 * 1e6
        }

        lastscavenge := nanotime()
        nscavenge := 0

        // Make wake-up period small enough for the sampling to be correct.
        maxsleep := forcegcperiod / 2
        if scavengelimit < forcegcperiod {
                maxsleep = scavengelimit / 2
        }

        lasttrace := int64(0)
        idle := 0 // how many cycles in succession we had not wokeup somebody
        delay := uint32(0)
        for {
                if idle == 0 { // start with 20us sleep...
                        delay = 20
                } else if idle > 50 { // start doubling the sleep after 1ms...
                        delay *= 2
                }
                if delay > 10*1000 { // up to 10ms
                        delay = 10 * 1000
                }
                usleep(delay)
                if debug.schedtrace <= 0 && (sched.gcwaiting != 0 || atomicload(&sched.npidle) == uint32(gomaxprocs)) { // TODO: fast atomic
                        lock(&sched.lock)
                        if atomicload(&sched.gcwaiting) != 0 || atomicload(&sched.npidle) == uint32(gomaxprocs) {
                                atomicstore(&sched.sysmonwait, 1)
                                unlock(&sched.lock)
                                notetsleep(&sched.sysmonnote, maxsleep)
                                lock(&sched.lock)
                                atomicstore(&sched.sysmonwait, 0)
                                noteclear(&sched.sysmonnote)
                                idle = 0
                                delay = 20
                        }
                        unlock(&sched.lock)
                }
                // poll network if not polled for more than 10ms
                lastpoll := int64(atomicload64(&sched.lastpoll))
                now := nanotime()
                unixnow := unixnanotime()
                if lastpoll != 0 && lastpoll+10*1000*1000 < now {
                        cas64(&sched.lastpoll, uint64(lastpoll), uint64(now))
                        gp := netpoll(false) // non-blocking - returns list of goroutines
                        if gp != nil {
                                // Need to decrement number of idle locked M's
                                // (pretending that one more is running) before injectglist.
                                // Otherwise it can lead to the following situation:
                                // injectglist grabs all P's but before it starts M's to run the P's,
                                // another M returns from syscall, finishes running its G,
                                // observes that there is no work to do and no other running M's
                                // and reports deadlock.
                                incidlelocked(-1)
                                injectglist(gp)
                                incidlelocked(1)
                        }
                }
                // retake P's blocked in syscalls
                // and preempt long running G's
                if retake(now) != 0 {
                        idle = 0
                } else {
                        idle++
                }
                // check if we need to force a GC
                lastgc := int64(atomicload64(&memstats.last_gc))
                if lastgc != 0 && unixnow-lastgc > forcegcperiod && atomicload(&forcegc.idle) != 0 {
                        lock(&forcegc.lock)
                        forcegc.idle = 0
                        forcegc.g.schedlink = nil
                        injectglist(forcegc.g)
                        unlock(&forcegc.lock)
                }
                // scavenge heap once in a while
                if lastscavenge+scavengelimit/2 < now {
                        mHeap_Scavenge(int32(nscavenge), uint64(now), uint64(scavengelimit))
                        lastscavenge = now
                        nscavenge++
                }
                if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace*1000000) <= now {
                        lasttrace = now
                        schedtrace(debug.scheddetail > 0)
                }
        }
}

var pdesc [_MaxGomaxprocs]struct {
        schedtick   uint32
        schedwhen   int64
        syscalltick uint32
        syscallwhen int64
}

func retake(now int64) uint32 {
        n := 0
        for i := int32(0); i < gomaxprocs; i++ {
                _p_ := allp[i]
                if _p_ == nil {
                        continue
                }
                pd := &pdesc[i]
                s := _p_.status
                if s == _Psyscall {
                        // Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
                        t := int64(_p_.syscalltick)
                        if int64(pd.syscalltick) != t {
                                pd.syscalltick = uint32(t)
                                pd.syscallwhen = now
                                continue
                        }
                        // On the one hand we don't want to retake Ps if there is no other work to do,
                        // but on the other hand we want to retake them eventually
                        // because they can prevent the sysmon thread from deep sleep.
                        if _p_.runqhead == _p_.runqtail && atomicload(&sched.nmspinning)+atomicload(&sched.npidle) > 0 && pd.syscallwhen+10*1000*1000 > now {
                                continue
                        }
                        // Need to decrement number of idle locked M's
                        // (pretending that one more is running) before the CAS.
                        // Otherwise the M from which we retake can exit the syscall,
                        // increment nmidle and report deadlock.
                        incidlelocked(-1)
                        if cas(&_p_.status, s, _Pidle) {
                                n++
                                handoffp(_p_)
                        }
                        incidlelocked(1)
                } else if s == _Prunning {
                        // Preempt G if it's running for more than 10ms.
                        t := int64(_p_.schedtick)
                        if int64(pd.schedtick) != t {
                                pd.schedtick = uint32(t)
                                pd.schedwhen = now
                                continue
                        }
                        if pd.schedwhen+10*1000*1000 > now {
                                continue
                        }
                        preemptone(_p_)
                }
        }
        return uint32(n)
}

// Tell all goroutines that they have been preempted and they should stop.
// This function is purely best-effort.  It can fail to inform a goroutine if a
// processor just started running it.
// No locks need to be held.
// Returns true if preemption request was issued to at least one goroutine.
func preemptall() bool {
        res := false
        for i := int32(0); i < gomaxprocs; i++ {
                _p_ := allp[i]
                if _p_ == nil || _p_.status != _Prunning {
                        continue
                }
                if preemptone(_p_) {
                        res = true
                }
        }
        return res
}

// Tell the goroutine running on processor P to stop.
// This function is purely best-effort.  It can incorrectly fail to inform the
// goroutine.  It can send inform the wrong goroutine.  Even if it informs the
// correct goroutine, that goroutine might ignore the request if it is
// simultaneously executing newstack.
// No lock needs to be held.
// Returns true if preemption request was issued.
// The actual preemption will happen at some point in the future
// and will be indicated by the gp->status no longer being
// Grunning
func preemptone(_p_ *p) bool {
        mp := _p_.m
        if mp == nil || mp == getg().m {
                return false
        }
        gp := mp.curg
        if gp == nil || gp == mp.g0 {
                return false
        }

        gp.preempt = true

        // Every call in a go routine checks for stack overflow by
        // comparing the current stack pointer to gp->stackguard0.
        // Setting gp->stackguard0 to StackPreempt folds
        // preemption into the normal stack overflow check.
        gp.stackguard0 = stackPreempt
        return true
}

var starttime int64

func schedtrace(detailed bool) {
        now := nanotime()
        if starttime == 0 {
                starttime = now
        }

        lock(&sched.lock)
        print("SCHED ", (now-starttime)/1e6, "ms: gomaxprocs=", gomaxprocs, " idleprocs=", sched.npidle, " threads=", sched.mcount, " spinningthreads=", sched.nmspinning, " idlethreads=", sched.nmidle, " runqueue=", sched.runqsize)
        if detailed {
                print(" gcwaiting=", sched.gcwaiting, " nmidlelocked=", sched.nmidlelocked, " stopwait=", sched.stopwait, " sysmonwait=", sched.sysmonwait, "\n")
        }
        // We must be careful while reading data from P's, M's and G's.
        // Even if we hold schedlock, most data can be changed concurrently.
        // E.g. (p->m ? p->m->id : -1) can crash if p->m changes from non-nil to nil.
        for i := int32(0); i < gomaxprocs; i++ {
                _p_ := allp[i]
                if _p_ == nil {
                        continue
                }
                mp := _p_.m
                h := atomicload(&_p_.runqhead)
                t := atomicload(&_p_.runqtail)
                if detailed {
                        id := int32(-1)
                        if mp != nil {
                                id = mp.id
                        }
                        print("  P", i, ": status=", _p_.status, " schedtick=", _p_.schedtick, " syscalltick=", _p_.syscalltick, " m=", id, " runqsize=", t-h, " gfreecnt=", _p_.gfreecnt, "\n")
                } else {
                        // In non-detailed mode format lengths of per-P run queues as:
                        // [len1 len2 len3 len4]
                        print(" ")
                        if i == 0 {
                                print("[")
                        }
                        print(t - h)
                        if i == gomaxprocs-1 {
                                print("]\n")
                        }
                }
        }

        if !detailed {
                unlock(&sched.lock)
                return
        }

        for mp := allm; mp != nil; mp = mp.alllink {
                _p_ := mp.p
                gp := mp.curg
                lockedg := mp.lockedg
                id1 := int32(-1)
                if _p_ != nil {
                        id1 = _p_.id
                }
                id2 := int64(-1)
                if gp != nil {
                        id2 = gp.goid
                }
                id3 := int64(-1)
                if lockedg != nil {
                        id3 = lockedg.goid
                }
                print("  M", mp.id, ": p=", id1, " curg=", id2, " mallocing=", mp.mallocing, " throwing=", mp.throwing, " gcing=", mp.gcing, ""+" locks=", mp.locks, " dying=", mp.dying, " helpgc=", mp.helpgc, " spinning=", mp.spinning, " blocked=", getg().m.blocked, " lockedg=", id3, "\n")
        }

        lock(&allglock)
        for gi := 0; gi < len(allgs); gi++ {
                gp := allgs[gi]
                mp := gp.m
                lockedm := gp.lockedm
                id1 := int32(-1)
                if mp != nil {
                        id1 = mp.id
                }
                id2 := int32(-1)
                if lockedm != nil {
                        id2 = lockedm.id
                }
                print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason, ") m=", id1, " lockedm=", id2, "\n")
        }
        unlock(&allglock)
        unlock(&sched.lock)
}

// Put mp on midle list.
// Sched must be locked.
func mput(mp *m) {
        mp.schedlink = sched.midle
        sched.midle = mp
        sched.nmidle++
        checkdead()
}

// Try to get an m from midle list.
// Sched must be locked.
func mget() *m {
        mp := sched.midle
        if mp != nil {
                sched.midle = mp.schedlink
                sched.nmidle--
        }
        return mp
}

// Put gp on the global runnable queue.
// Sched must be locked.
func globrunqput(gp *g) {
        gp.schedlink = nil
        if sched.runqtail != nil {
                sched.runqtail.schedlink = gp
        } else {
                sched.runqhead = gp
        }
        sched.runqtail = gp
        sched.runqsize++
}

// Put a batch of runnable goroutines on the global runnable queue.
// Sched must be locked.
func globrunqputbatch(ghead *g, gtail *g, n int32) {
        gtail.schedlink = nil
        if sched.runqtail != nil {
                sched.runqtail.schedlink = ghead
        } else {
                sched.runqhead = ghead
        }
        sched.runqtail = gtail
        sched.runqsize += n
}

// Try get a batch of G's from the global runnable queue.
// Sched must be locked.
func globrunqget(_p_ *p, max int32) *g {
        if sched.runqsize == 0 {
                return nil
        }

        n := sched.runqsize/gomaxprocs + 1
        if n > sched.runqsize {
                n = sched.runqsize
        }
        if max > 0 && n > max {
                n = max
        }
        if n > int32(len(_p_.runq))/2 {
                n = int32(len(_p_.runq)) / 2
        }

        sched.runqsize -= n
        if sched.runqsize == 0 {
                sched.runqtail = nil
        }

        gp := sched.runqhead
        sched.runqhead = gp.schedlink
        n--
        for ; n > 0; n-- {
                gp1 := sched.runqhead
                sched.runqhead = gp1.schedlink
                runqput(_p_, gp1)
        }
        return gp
}

// Put p to on _Pidle list.
// Sched must be locked.
func pidleput(_p_ *p) {
        _p_.link = sched.pidle
        sched.pidle = _p_
        xadd(&sched.npidle, 1) // TODO: fast atomic
}

// Try get a p from _Pidle list.
// Sched must be locked.
func pidleget() *p {
        _p_ := sched.pidle
        if _p_ != nil {
                sched.pidle = _p_.link
                xadd(&sched.npidle, -1) // TODO: fast atomic
        }
        return _p_
}

// Try to put g on local runnable queue.
// If it's full, put onto global queue.
// Executed only by the owner P.
func runqput(_p_ *p, gp *g) {
retry:
        h := atomicload(&_p_.runqhead) // load-acquire, synchronize with consumers
        t := _p_.runqtail
        if t-h < uint32(len(_p_.runq)) {
                _p_.runq[t%uint32(len(_p_.runq))] = gp
                atomicstore(&_p_.runqtail, t+1) // store-release, makes the item available for consumption
                return
        }
        if runqputslow(_p_, gp, h, t) {
                return
        }
        // the queue is not full, now the put above must suceed
        goto retry
}

// Put g and a batch of work from local runnable queue on global queue.
// Executed only by the owner P.
func runqputslow(_p_ *p, gp *g, h, t uint32) bool {
        var batch [len(_p_.runq)/2 + 1]*g

        // First, grab a batch from local queue.
        n := t - h
        n = n / 2
        if n != uint32(len(_p_.runq)/2) {
                gothrow("runqputslow: queue is not full")
        }
        for i := uint32(0); i < n; i++ {
                batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))]
        }
        if !cas(&_p_.runqhead, h, h+n) { // cas-release, commits consume
                return false
        }
        batch[n] = gp

        // Link the goroutines.
        for i := uint32(0); i < n; i++ {
                batch[i].schedlink = batch[i+1]
        }

        // Now put the batch on global queue.
        lock(&sched.lock)
        globrunqputbatch(batch[0], batch[n], int32(n+1))
        unlock(&sched.lock)
        return true
}

// Get g from local runnable queue.
// Executed only by the owner P.
func runqget(_p_ *p) *g {
        for {
                h := atomicload(&_p_.runqhead) // load-acquire, synchronize with other consumers
                t := _p_.runqtail
                if t == h {
                        return nil
                }
                gp := _p_.runq[h%uint32(len(_p_.runq))]
                if cas(&_p_.runqhead, h, h+1) { // cas-release, commits consume
                        return gp
                }
        }
}

// Grabs a batch of goroutines from local runnable queue.
// batch array must be of size len(p->runq)/2. Returns number of grabbed goroutines.
// Can be executed by any P.
func runqgrab(_p_ *p, batch []*g) uint32 {
        for {
                h := atomicload(&_p_.runqhead) // load-acquire, synchronize with other consumers
                t := atomicload(&_p_.runqtail) // load-acquire, synchronize with the producer
                n := t - h
                n = n - n/2
                if n == 0 {
                        return 0
                }
                if n > uint32(len(_p_.runq)/2) { // read inconsistent h and t
                        continue
                }
                for i := uint32(0); i < n; i++ {
                        batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))]
                }
                if cas(&_p_.runqhead, h, h+n) { // cas-release, commits consume
                        return n
                }
        }
}

// Steal half of elements from local runnable queue of p2
// and put onto local runnable queue of p.
// Returns one of the stolen elements (or nil if failed).
func runqsteal(_p_, p2 *p) *g {
        var batch [len(_p_.runq) / 2]*g

        n := runqgrab(p2, batch[:])
        if n == 0 {
                return nil
        }
        n--
        gp := batch[n]
        if n == 0 {
                return gp
        }
        h := atomicload(&_p_.runqhead) // load-acquire, synchronize with consumers
        t := _p_.runqtail
        if t-h+n >= uint32(len(_p_.runq)) {
                gothrow("runqsteal: runq overflow")
        }
        for i := uint32(0); i < n; i++ {
                _p_.runq[(t+i)%uint32(len(_p_.runq))] = batch[i]
        }
        atomicstore(&_p_.runqtail, t+n) // store-release, makes the item available for consumption
        return gp
}

func testSchedLocalQueue() {
        _p_ := new(p)
        gs := make([]g, len(_p_.runq))
        for i := 0; i < len(_p_.runq); i++ {
                if runqget(_p_) != nil {
                        gothrow("runq is not empty initially")
                }
                for j := 0; j < i; j++ {
                        runqput(_p_, &gs[i])
                }
                for j := 0; j < i; j++ {
                        if runqget(_p_) != &gs[i] {
                                print("bad element at iter ", i, "/", j, "\n")
                                gothrow("bad element")
                        }
                }
                if runqget(_p_) != nil {
                        gothrow("runq is not empty afterwards")
                }
        }
}

func testSchedLocalQueueSteal() {
        p1 := new(p)
        p2 := new(p)
        gs := make([]g, len(p1.runq))
        for i := 0; i < len(p1.runq); i++ {
                for j := 0; j < i; j++ {
                        gs[j].sig = 0
                        runqput(p1, &gs[j])
                }
                gp := runqsteal(p2, p1)
                s := 0
                if gp != nil {
                        s++
                        gp.sig++
                }
                for {
                        gp = runqget(p2)
                        if gp == nil {
                                break
                        }
                        s++
                        gp.sig++
                }
                for {
                        gp = runqget(p1)
                        if gp == nil {
                                break
                        }
                        gp.sig++
                }
                for j := 0; j < i; j++ {
                        if gs[j].sig != 1 {
                                print("bad element ", j, "(", gs[j].sig, ") at iter ", i, "\n")
                                gothrow("bad element")
                        }
                }
                if s != i/2 && s != i/2+1 {
                        print("bad steal ", s, ", want ", i/2, " or ", i/2+1, ", iter ", i, "\n")
                        gothrow("bad steal")
                }
        }
}

func setMaxThreads(in int) (out int) {
        lock(&sched.lock)
        out = int(sched.maxmcount)
        sched.maxmcount = int32(in)
        checkmcount()
        unlock(&sched.lock)
        return
}

var goexperiment string = "GOEXPERIMENT" // TODO: defined in zaexperiment.h

func haveexperiment(name string) bool {
        x := goexperiment
        for x != "" {
                xname := ""
                i := index(x, ",")
                if i < 0 {
                        xname, x = x, ""
                } else {
                        xname, x = x[:i], x[i+1:]
                }
                if xname == name {
                        return true
                }
        }
        return false
}

//go:nosplit
func sync_procPin() int {
        _g_ := getg()
        mp := _g_.m

        mp.locks++
        return int(mp.p.id)
}

//go:nosplit
func sync_procUnpin() {
        _g_ := getg()
        _g_.m.locks--
}