runtime: track the amount of scannable allocated stack for the GC pacer

[gostls13.git] / src / runtime / runtime2.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 (
        "internal/goarch"
        "runtime/internal/atomic"
        "unsafe"
)

// defined constants
const (
        // G status
        //
        // Beyond indicating the general state of a G, the G status
        // acts like a lock on the goroutine's stack (and hence its
        // ability to execute user code).
        //
        // If you add to this list, add to the list
        // of "okay during garbage collection" status
        // in mgcmark.go too.
        //
        // TODO(austin): The _Gscan bit could be much lighter-weight.
        // For example, we could choose not to run _Gscanrunnable
        // goroutines found in the run queue, rather than CAS-looping
        // until they become _Grunnable. And transitions like
        // _Gscanwaiting -> _Gscanrunnable are actually okay because
        // they don't affect stack ownership.

        // _Gidle means this goroutine was just allocated and has not
        // yet been initialized.
        _Gidle = iota // 0

        // _Grunnable means this goroutine is on a run queue. It is
        // not currently executing user code. The stack is not owned.
        _Grunnable // 1

        // _Grunning means this goroutine may execute user code. The
        // stack is owned by this goroutine. It is not on a run queue.
        // It is assigned an M and a P (g.m and g.m.p are valid).
        _Grunning // 2

        // _Gsyscall means this goroutine is executing a system call.
        // It is not executing user code. The stack is owned by this
        // goroutine. It is not on a run queue. It is assigned an M.
        _Gsyscall // 3

        // _Gwaiting means this goroutine is blocked in the runtime.
        // It is not executing user code. It is not on a run queue,
        // but should be recorded somewhere (e.g., a channel wait
        // queue) so it can be ready()d when necessary. The stack is
        // not owned *except* that a channel operation may read or
        // write parts of the stack under the appropriate channel
        // lock. Otherwise, it is not safe to access the stack after a
        // goroutine enters _Gwaiting (e.g., it may get moved).
        _Gwaiting // 4

        // _Gmoribund_unused is currently unused, but hardcoded in gdb
        // scripts.
        _Gmoribund_unused // 5

        // _Gdead means this goroutine is currently unused. It may be
        // just exited, on a free list, or just being initialized. It
        // is not executing user code. It may or may not have a stack
        // allocated. The G and its stack (if any) are owned by the M
        // that is exiting the G or that obtained the G from the free
        // list.
        _Gdead // 6

        // _Genqueue_unused is currently unused.
        _Genqueue_unused // 7

        // _Gcopystack means this goroutine's stack is being moved. It
        // is not executing user code and is not on a run queue. The
        // stack is owned by the goroutine that put it in _Gcopystack.
        _Gcopystack // 8

        // _Gpreempted means this goroutine stopped itself for a
        // suspendG preemption. It is like _Gwaiting, but nothing is
        // yet responsible for ready()ing it. Some suspendG must CAS
        // the status to _Gwaiting to take responsibility for
        // ready()ing this G.
        _Gpreempted // 9

        // _Gscan combined with one of the above states other than
        // _Grunning indicates that GC is scanning the stack. The
        // goroutine is not executing user code and the stack is owned
        // by the goroutine that set the _Gscan bit.
        //
        // _Gscanrunning is different: it is used to briefly block
        // state transitions while GC signals the G to scan its own
        // stack. This is otherwise like _Grunning.
        //
        // atomicstatus&~Gscan gives the state the goroutine will
        // return to when the scan completes.
        _Gscan          = 0x1000
        _Gscanrunnable  = _Gscan + _Grunnable  // 0x1001
        _Gscanrunning   = _Gscan + _Grunning   // 0x1002
        _Gscansyscall   = _Gscan + _Gsyscall   // 0x1003
        _Gscanwaiting   = _Gscan + _Gwaiting   // 0x1004
        _Gscanpreempted = _Gscan + _Gpreempted // 0x1009
)

const (
        // P status

        // _Pidle means a P is not being used to run user code or the
        // scheduler. Typically, it's on the idle P list and available
        // to the scheduler, but it may just be transitioning between
        // other states.
        //
        // The P is owned by the idle list or by whatever is
        // transitioning its state. Its run queue is empty.
        _Pidle = iota

        // _Prunning means a P is owned by an M and is being used to
        // run user code or the scheduler. Only the M that owns this P
        // is allowed to change the P's status from _Prunning. The M
        // may transition the P to _Pidle (if it has no more work to
        // do), _Psyscall (when entering a syscall), or _Pgcstop (to
        // halt for the GC). The M may also hand ownership of the P
        // off directly to another M (e.g., to schedule a locked G).
        _Prunning

        // _Psyscall means a P is not running user code. It has
        // affinity to an M in a syscall but is not owned by it and
        // may be stolen by another M. This is similar to _Pidle but
        // uses lightweight transitions and maintains M affinity.
        //
        // Leaving _Psyscall must be done with a CAS, either to steal
        // or retake the P. Note that there's an ABA hazard: even if
        // an M successfully CASes its original P back to _Prunning
        // after a syscall, it must understand the P may have been
        // used by another M in the interim.
        _Psyscall

        // _Pgcstop means a P is halted for STW and owned by the M
        // that stopped the world. The M that stopped the world
        // continues to use its P, even in _Pgcstop. Transitioning
        // from _Prunning to _Pgcstop causes an M to release its P and
        // park.
        //
        // The P retains its run queue and startTheWorld will restart
        // the scheduler on Ps with non-empty run queues.
        _Pgcstop

        // _Pdead means a P is no longer used (GOMAXPROCS shrank). We
        // reuse Ps if GOMAXPROCS increases. A dead P is mostly
        // stripped of its resources, though a few things remain
        // (e.g., trace buffers).
        _Pdead
)

// Mutual exclusion locks.  In the uncontended case,
// as fast as spin locks (just a few user-level instructions),
// but on the contention path they sleep in the kernel.
// A zeroed Mutex is unlocked (no need to initialize each lock).
// Initialization is helpful for static lock ranking, but not required.
type mutex struct {
        // Empty struct if lock ranking is disabled, otherwise includes the lock rank
        lockRankStruct
        // Futex-based impl treats it as uint32 key,
        // while sema-based impl as M* waitm.
        // Used to be a union, but unions break precise GC.
        key uintptr
}

// sleep and wakeup on one-time events.
// before any calls to notesleep or notewakeup,
// must call noteclear to initialize the Note.
// then, exactly one thread can call notesleep
// and exactly one thread can call notewakeup (once).
// once notewakeup has been called, the notesleep
// will return.  future notesleep will return immediately.
// subsequent noteclear must be called only after
// previous notesleep has returned, e.g. it's disallowed
// to call noteclear straight after notewakeup.
//
// notetsleep is like notesleep but wakes up after
// a given number of nanoseconds even if the event
// has not yet happened.  if a goroutine uses notetsleep to
// wake up early, it must wait to call noteclear until it
// can be sure that no other goroutine is calling
// notewakeup.
//
// notesleep/notetsleep are generally called on g0,
// notetsleepg is similar to notetsleep but is called on user g.
type note struct {
        // Futex-based impl treats it as uint32 key,
        // while sema-based impl as M* waitm.
        // Used to be a union, but unions break precise GC.
        key uintptr
}

type funcval struct {
        fn uintptr
        // variable-size, fn-specific data here
}

type iface struct {
        tab  *itab
        data unsafe.Pointer
}

type eface struct {
        _type *_type
        data  unsafe.Pointer
}

func efaceOf(ep *interface{}) *eface {
        return (*eface)(unsafe.Pointer(ep))
}

// The guintptr, muintptr, and puintptr are all used to bypass write barriers.
// It is particularly important to avoid write barriers when the current P has
// been released, because the GC thinks the world is stopped, and an
// unexpected write barrier would not be synchronized with the GC,
// which can lead to a half-executed write barrier that has marked the object
// but not queued it. If the GC skips the object and completes before the
// queuing can occur, it will incorrectly free the object.
//
// We tried using special assignment functions invoked only when not
// holding a running P, but then some updates to a particular memory
// word went through write barriers and some did not. This breaks the
// write barrier shadow checking mode, and it is also scary: better to have
// a word that is completely ignored by the GC than to have one for which
// only a few updates are ignored.
//
// Gs and Ps are always reachable via true pointers in the
// allgs and allp lists or (during allocation before they reach those lists)
// from stack variables.
//
// Ms are always reachable via true pointers either from allm or
// freem. Unlike Gs and Ps we do free Ms, so it's important that
// nothing ever hold an muintptr across a safe point.

// A guintptr holds a goroutine pointer, but typed as a uintptr
// to bypass write barriers. It is used in the Gobuf goroutine state
// and in scheduling lists that are manipulated without a P.
//
// The Gobuf.g goroutine pointer is almost always updated by assembly code.
// In one of the few places it is updated by Go code - func save - it must be
// treated as a uintptr to avoid a write barrier being emitted at a bad time.
// Instead of figuring out how to emit the write barriers missing in the
// assembly manipulation, we change the type of the field to uintptr,
// so that it does not require write barriers at all.
//
// Goroutine structs are published in the allg list and never freed.
// That will keep the goroutine structs from being collected.
// There is never a time that Gobuf.g's contain the only references
// to a goroutine: the publishing of the goroutine in allg comes first.
// Goroutine pointers are also kept in non-GC-visible places like TLS,
// so I can't see them ever moving. If we did want to start moving data
// in the GC, we'd need to allocate the goroutine structs from an
// alternate arena. Using guintptr doesn't make that problem any worse.
type guintptr uintptr

//go:nosplit
func (gp guintptr) ptr() *g { return (*g)(unsafe.Pointer(gp)) }

//go:nosplit
func (gp *guintptr) set(g *g) { *gp = guintptr(unsafe.Pointer(g)) }

//go:nosplit
func (gp *guintptr) cas(old, new guintptr) bool {
        return atomic.Casuintptr((*uintptr)(unsafe.Pointer(gp)), uintptr(old), uintptr(new))
}

// setGNoWB performs *gp = new without a write barrier.
// For times when it's impractical to use a guintptr.
//go:nosplit
//go:nowritebarrier
func setGNoWB(gp **g, new *g) {
        (*guintptr)(unsafe.Pointer(gp)).set(new)
}

type puintptr uintptr

//go:nosplit
func (pp puintptr) ptr() *p { return (*p)(unsafe.Pointer(pp)) }

//go:nosplit
func (pp *puintptr) set(p *p) { *pp = puintptr(unsafe.Pointer(p)) }

// muintptr is a *m that is not tracked by the garbage collector.
//
// Because we do free Ms, there are some additional constrains on
// muintptrs:
//
// 1. Never hold an muintptr locally across a safe point.
//
// 2. Any muintptr in the heap must be owned by the M itself so it can
//    ensure it is not in use when the last true *m is released.
type muintptr uintptr

//go:nosplit
func (mp muintptr) ptr() *m { return (*m)(unsafe.Pointer(mp)) }

//go:nosplit
func (mp *muintptr) set(m *m) { *mp = muintptr(unsafe.Pointer(m)) }

// setMNoWB performs *mp = new without a write barrier.
// For times when it's impractical to use an muintptr.
//go:nosplit
//go:nowritebarrier
func setMNoWB(mp **m, new *m) {
        (*muintptr)(unsafe.Pointer(mp)).set(new)
}

type gobuf struct {
        // The offsets of sp, pc, and g are known to (hard-coded in) libmach.
        //
        // ctxt is unusual with respect to GC: it may be a
        // heap-allocated funcval, so GC needs to track it, but it
        // needs to be set and cleared from assembly, where it's
        // difficult to have write barriers. However, ctxt is really a
        // saved, live register, and we only ever exchange it between
        // the real register and the gobuf. Hence, we treat it as a
        // root during stack scanning, which means assembly that saves
        // and restores it doesn't need write barriers. It's still
        // typed as a pointer so that any other writes from Go get
        // write barriers.
        sp   uintptr
        pc   uintptr
        g    guintptr
        ctxt unsafe.Pointer
        ret  uintptr
        lr   uintptr
        bp   uintptr // for framepointer-enabled architectures
}

// sudog represents a g in a wait list, such as for sending/receiving
// on a channel.
//
// sudog is necessary because the g ↔ synchronization object relation
// is many-to-many. A g can be on many wait lists, so there may be
// many sudogs for one g; and many gs may be waiting on the same
// synchronization object, so there may be many sudogs for one object.
//
// sudogs are allocated from a special pool. Use acquireSudog and
// releaseSudog to allocate and free them.
type sudog struct {
        // The following fields are protected by the hchan.lock of the
        // channel this sudog is blocking on. shrinkstack depends on
        // this for sudogs involved in channel ops.

        g *g

        next *sudog
        prev *sudog
        elem unsafe.Pointer // data element (may point to stack)

        // The following fields are never accessed concurrently.
        // For channels, waitlink is only accessed by g.
        // For semaphores, all fields (including the ones above)
        // are only accessed when holding a semaRoot lock.

        acquiretime int64
        releasetime int64
        ticket      uint32

        // isSelect indicates g is participating in a select, so
        // g.selectDone must be CAS'd to win the wake-up race.
        isSelect bool

        // success indicates whether communication over channel c
        // succeeded. It is true if the goroutine was awoken because a
        // value was delivered over channel c, and false if awoken
        // because c was closed.
        success bool

        parent   *sudog // semaRoot binary tree
        waitlink *sudog // g.waiting list or semaRoot
        waittail *sudog // semaRoot
        c        *hchan // channel
}

type libcall struct {
        fn   uintptr
        n    uintptr // number of parameters
        args uintptr // parameters
        r1   uintptr // return values
        r2   uintptr
        err  uintptr // error number
}

// Stack describes a Go execution stack.
// The bounds of the stack are exactly [lo, hi),
// with no implicit data structures on either side.
type stack struct {
        lo uintptr
        hi uintptr
}

// heldLockInfo gives info on a held lock and the rank of that lock
type heldLockInfo struct {
        lockAddr uintptr
        rank     lockRank
}

type g struct {
        // Stack parameters.
        // stack describes the actual stack memory: [stack.lo, stack.hi).
        // stackguard0 is the stack pointer compared in the Go stack growth prologue.
        // It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
        // stackguard1 is the stack pointer compared in the C stack growth prologue.
        // It is stack.lo+StackGuard on g0 and gsignal stacks.
        // It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
        stack       stack   // offset known to runtime/cgo
        stackguard0 uintptr // offset known to liblink
        stackguard1 uintptr // offset known to liblink

        _panic    *_panic // innermost panic - offset known to liblink
        _defer    *_defer // innermost defer
        m         *m      // current m; offset known to arm liblink
        sched     gobuf
        syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
        syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
        stktopsp  uintptr // expected sp at top of stack, to check in traceback
        // param is a generic pointer parameter field used to pass
        // values in particular contexts where other storage for the
        // parameter would be difficult to find. It is currently used
        // in three ways:
        // 1. When a channel operation wakes up a blocked goroutine, it sets param to
        //    point to the sudog of the completed blocking operation.
        // 2. By gcAssistAlloc1 to signal back to its caller that the goroutine completed
        //    the GC cycle. It is unsafe to do so in any other way, because the goroutine's
        //    stack may have moved in the meantime.
        // 3. By debugCallWrap to pass parameters to a new goroutine because allocating a
        //    closure in the runtime is forbidden.
        param        unsafe.Pointer
        atomicstatus uint32
        stackLock    uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
        goid         int64
        schedlink    guintptr
        waitsince    int64      // approx time when the g become blocked
        waitreason   waitReason // if status==Gwaiting

        preempt       bool // preemption signal, duplicates stackguard0 = stackpreempt
        preemptStop   bool // transition to _Gpreempted on preemption; otherwise, just deschedule
        preemptShrink bool // shrink stack at synchronous safe point

        // asyncSafePoint is set if g is stopped at an asynchronous
        // safe point. This means there are frames on the stack
        // without precise pointer information.
        asyncSafePoint bool

        paniconfault bool // panic (instead of crash) on unexpected fault address
        gcscandone   bool // g has scanned stack; protected by _Gscan bit in status
        throwsplit   bool // must not split stack
        // activeStackChans indicates that there are unlocked channels
        // pointing into this goroutine's stack. If true, stack
        // copying needs to acquire channel locks to protect these
        // areas of the stack.
        activeStackChans bool
        // parkingOnChan indicates that the goroutine is about to
        // park on a chansend or chanrecv. Used to signal an unsafe point
        // for stack shrinking. It's a boolean value, but is updated atomically.
        parkingOnChan uint8

        raceignore     int8     // ignore race detection events
        sysblocktraced bool     // StartTrace has emitted EvGoInSyscall about this goroutine
        tracking       bool     // whether we're tracking this G for sched latency statistics
        trackingSeq    uint8    // used to decide whether to track this G
        runnableStamp  int64    // timestamp of when the G last became runnable, only used when tracking
        runnableTime   int64    // the amount of time spent runnable, cleared when running, only used when tracking
        sysexitticks   int64    // cputicks when syscall has returned (for tracing)
        traceseq       uint64   // trace event sequencer
        tracelastp     puintptr // last P emitted an event for this goroutine
        lockedm        muintptr
        sig            uint32
        writebuf       []byte
        sigcode0       uintptr
        sigcode1       uintptr
        sigpc          uintptr
        gopc           uintptr         // pc of go statement that created this goroutine
        ancestors      *[]ancestorInfo // ancestor information goroutine(s) that created this goroutine (only used if debug.tracebackancestors)
        startpc        uintptr         // pc of goroutine function
        racectx        uintptr
        waiting        *sudog         // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
        cgoCtxt        []uintptr      // cgo traceback context
        labels         unsafe.Pointer // profiler labels
        timer          *timer         // cached timer for time.Sleep
        selectDone     uint32         // are we participating in a select and did someone win the race?

        // Per-G GC state

        // gcAssistBytes is this G's GC assist credit in terms of
        // bytes allocated. If this is positive, then the G has credit
        // to allocate gcAssistBytes bytes without assisting. If this
        // is negative, then the G must correct this by performing
        // scan work. We track this in bytes to make it fast to update
        // and check for debt in the malloc hot path. The assist ratio
        // determines how this corresponds to scan work debt.
        gcAssistBytes int64
}

// gTrackingPeriod is the number of transitions out of _Grunning between
// latency tracking runs.
const gTrackingPeriod = 8

const (
        // tlsSlots is the number of pointer-sized slots reserved for TLS on some platforms,
        // like Windows.
        tlsSlots = 6
        tlsSize  = tlsSlots * goarch.PtrSize
)

type m struct {
        g0      *g     // goroutine with scheduling stack
        morebuf gobuf  // gobuf arg to morestack
        divmod  uint32 // div/mod denominator for arm - known to liblink

        // Fields not known to debuggers.
        procid        uint64            // for debuggers, but offset not hard-coded
        gsignal       *g                // signal-handling g
        goSigStack    gsignalStack      // Go-allocated signal handling stack
        sigmask       sigset            // storage for saved signal mask
        tls           [tlsSlots]uintptr // thread-local storage (for x86 extern register)
        mstartfn      func()
        curg          *g       // current running goroutine
        caughtsig     guintptr // goroutine running during fatal signal
        p             puintptr // attached p for executing go code (nil if not executing go code)
        nextp         puintptr
        oldp          puintptr // the p that was attached before executing a syscall
        id            int64
        mallocing     int32
        throwing      int32
        preemptoff    string // if != "", keep curg running on this m
        locks         int32
        dying         int32
        profilehz     int32
        spinning      bool // m is out of work and is actively looking for work
        blocked       bool // m is blocked on a note
        newSigstack   bool // minit on C thread called sigaltstack
        printlock     int8
        incgo         bool   // m is executing a cgo call
        freeWait      uint32 // if == 0, safe to free g0 and delete m (atomic)
        fastrand      uint64
        needextram    bool
        traceback     uint8
        ncgocall      uint64      // number of cgo calls in total
        ncgo          int32       // number of cgo calls currently in progress
        cgoCallersUse uint32      // if non-zero, cgoCallers in use temporarily
        cgoCallers    *cgoCallers // cgo traceback if crashing in cgo call
        doesPark      bool        // non-P running threads: sysmon and newmHandoff never use .park
        park          note
        alllink       *m // on allm
        schedlink     muintptr
        lockedg       guintptr
        createstack   [32]uintptr // stack that created this thread.
        lockedExt     uint32      // tracking for external LockOSThread
        lockedInt     uint32      // tracking for internal lockOSThread
        nextwaitm     muintptr    // next m waiting for lock
        waitunlockf   func(*g, unsafe.Pointer) bool
        waitlock      unsafe.Pointer
        waittraceev   byte
        waittraceskip int
        startingtrace bool
        syscalltick   uint32
        freelink      *m // on sched.freem

        // mFixup is used to synchronize OS related m state
        // (credentials etc) use mutex to access. To avoid deadlocks
        // an atomic.Load() of used being zero in mDoFixupFn()
        // guarantees fn is nil.
        mFixup struct {
                lock mutex
                used uint32
                fn   func(bool) bool
        }

        // these are here because they are too large to be on the stack
        // of low-level NOSPLIT functions.
        libcall   libcall
        libcallpc uintptr // for cpu profiler
        libcallsp uintptr
        libcallg  guintptr
        syscall   libcall // stores syscall parameters on windows

        vdsoSP uintptr // SP for traceback while in VDSO call (0 if not in call)
        vdsoPC uintptr // PC for traceback while in VDSO call

        // preemptGen counts the number of completed preemption
        // signals. This is used to detect when a preemption is
        // requested, but fails. Accessed atomically.
        preemptGen uint32

        // Whether this is a pending preemption signal on this M.
        // Accessed atomically.
        signalPending uint32

        dlogPerM

        mOS

        // Up to 10 locks held by this m, maintained by the lock ranking code.
        locksHeldLen int
        locksHeld    [10]heldLockInfo
}

type p struct {
        id          int32
        status      uint32 // one of pidle/prunning/...
        link        puintptr
        schedtick   uint32     // incremented on every scheduler call
        syscalltick uint32     // incremented on every system call
        sysmontick  sysmontick // last tick observed by sysmon
        m           muintptr   // back-link to associated m (nil if idle)
        mcache      *mcache
        pcache      pageCache
        raceprocctx uintptr

        deferpool    []*_defer // pool of available defer structs (see panic.go)
        deferpoolbuf [32]*_defer

        // Cache of goroutine ids, amortizes accesses to runtime·sched.goidgen.
        goidcache    uint64
        goidcacheend uint64

        // Queue of runnable goroutines. Accessed without lock.
        runqhead uint32
        runqtail uint32
        runq     [256]guintptr
        // runnext, if non-nil, is a runnable G that was ready'd by
        // the current G and should be run next instead of what's in
        // runq if there's time remaining in the running G's time
        // slice. It will inherit the time left in the current time
        // slice. If a set of goroutines is locked in a
        // communicate-and-wait pattern, this schedules that set as a
        // unit and eliminates the (potentially large) scheduling
        // latency that otherwise arises from adding the ready'd
        // goroutines to the end of the run queue.
        //
        // Note that while other P's may atomically CAS this to zero,
        // only the owner P can CAS it to a valid G.
        runnext guintptr

        // Available G's (status == Gdead)
        gFree struct {
                gList
                n int32
        }

        sudogcache []*sudog
        sudogbuf   [128]*sudog

        // Cache of mspan objects from the heap.
        mspancache struct {
                // We need an explicit length here because this field is used
                // in allocation codepaths where write barriers are not allowed,
                // and eliminating the write barrier/keeping it eliminated from
                // slice updates is tricky, moreso than just managing the length
                // ourselves.
                len int
                buf [128]*mspan
        }

        tracebuf traceBufPtr

        // traceSweep indicates the sweep events should be traced.
        // This is used to defer the sweep start event until a span
        // has actually been swept.
        traceSweep bool
        // traceSwept and traceReclaimed track the number of bytes
        // swept and reclaimed by sweeping in the current sweep loop.
        traceSwept, traceReclaimed uintptr

        palloc persistentAlloc // per-P to avoid mutex

        _ uint32 // Alignment for atomic fields below

        // The when field of the first entry on the timer heap.
        // This is updated using atomic functions.
        // This is 0 if the timer heap is empty.
        timer0When uint64

        // The earliest known nextwhen field of a timer with
        // timerModifiedEarlier status. Because the timer may have been
        // modified again, there need not be any timer with this value.
        // This is updated using atomic functions.
        // This is 0 if there are no timerModifiedEarlier timers.
        timerModifiedEarliest uint64

        // Per-P GC state
        gcAssistTime         int64 // Nanoseconds in assistAlloc
        gcFractionalMarkTime int64 // Nanoseconds in fractional mark worker (atomic)

        // gcMarkWorkerMode is the mode for the next mark worker to run in.
        // That is, this is used to communicate with the worker goroutine
        // selected for immediate execution by
        // gcController.findRunnableGCWorker. When scheduling other goroutines,
        // this field must be set to gcMarkWorkerNotWorker.
        gcMarkWorkerMode gcMarkWorkerMode
        // gcMarkWorkerStartTime is the nanotime() at which the most recent
        // mark worker started.
        gcMarkWorkerStartTime int64

        // gcw is this P's GC work buffer cache. The work buffer is
        // filled by write barriers, drained by mutator assists, and
        // disposed on certain GC state transitions.
        gcw gcWork

        // wbBuf is this P's GC write barrier buffer.
        //
        // TODO: Consider caching this in the running G.
        wbBuf wbBuf

        runSafePointFn uint32 // if 1, run sched.safePointFn at next safe point

        // statsSeq is a counter indicating whether this P is currently
        // writing any stats. Its value is even when not, odd when it is.
        statsSeq uint32

        // Lock for timers. We normally access the timers while running
        // on this P, but the scheduler can also do it from a different P.
        timersLock mutex

        // Actions to take at some time. This is used to implement the
        // standard library's time package.
        // Must hold timersLock to access.
        timers []*timer

        // Number of timers in P's heap.
        // Modified using atomic instructions.
        numTimers uint32

        // Number of timerDeleted timers in P's heap.
        // Modified using atomic instructions.
        deletedTimers uint32

        // Race context used while executing timer functions.
        timerRaceCtx uintptr

        // scannableStackSizeDelta accumulates the amount of stack space held by
        // live goroutines (i.e. those eligible for stack scanning).
        // Flushed to gcController.scannableStackSize once scannableStackSizeSlack
        // or -scannableStackSizeSlack is reached.
        scannableStackSizeDelta int64

        // preempt is set to indicate that this P should be enter the
        // scheduler ASAP (regardless of what G is running on it).
        preempt bool

        // Padding is no longer needed. False sharing is now not a worry because p is large enough
        // that its size class is an integer multiple of the cache line size (for any of our architectures).
}

type schedt struct {
        // accessed atomically. keep at top to ensure alignment on 32-bit systems.
        goidgen   uint64
        lastpoll  uint64 // time of last network poll, 0 if currently polling
        pollUntil uint64 // time to which current poll is sleeping

        lock mutex

        // When increasing nmidle, nmidlelocked, nmsys, or nmfreed, be
        // sure to call checkdead().

        midle        muintptr // idle m's waiting for work
        nmidle       int32    // number of idle m's waiting for work
        nmidlelocked int32    // number of locked m's waiting for work
        mnext        int64    // number of m's that have been created and next M ID
        maxmcount    int32    // maximum number of m's allowed (or die)
        nmsys        int32    // number of system m's not counted for deadlock
        nmfreed      int64    // cumulative number of freed m's

        ngsys uint32 // number of system goroutines; updated atomically

        pidle      puintptr // idle p's
        npidle     uint32
        nmspinning uint32 // See "Worker thread parking/unparking" comment in proc.go.

        // Global runnable queue.
        runq     gQueue
        runqsize int32

        // disable controls selective disabling of the scheduler.
        //
        // Use schedEnableUser to control this.
        //
        // disable is protected by sched.lock.
        disable struct {
                // user disables scheduling of user goroutines.
                user     bool
                runnable gQueue // pending runnable Gs
                n        int32  // length of runnable
        }

        // Global cache of dead G's.
        gFree struct {
                lock    mutex
                stack   gList // Gs with stacks
                noStack gList // Gs without stacks
                n       int32
        }

        // Central cache of sudog structs.
        sudoglock  mutex
        sudogcache *sudog

        // Central pool of available defer structs.
        deferlock mutex
        deferpool *_defer

        // freem is the list of m's waiting to be freed when their
        // m.exited is set. Linked through m.freelink.
        freem *m

        gcwaiting  uint32 // gc is waiting to run
        stopwait   int32
        stopnote   note
        sysmonwait uint32
        sysmonnote note

        // While true, sysmon not ready for mFixup calls.
        // Accessed atomically.
        sysmonStarting uint32

        // safepointFn should be called on each P at the next GC
        // safepoint if p.runSafePointFn is set.
        safePointFn   func(*p)
        safePointWait int32
        safePointNote note

        profilehz int32 // cpu profiling rate

        procresizetime int64 // nanotime() of last change to gomaxprocs
        totaltime      int64 // ∫gomaxprocs dt up to procresizetime

        // sysmonlock protects sysmon's actions on the runtime.
        //
        // Acquire and hold this mutex to block sysmon from interacting
        // with the rest of the runtime.
        sysmonlock mutex

        _ uint32 // ensure timeToRun has 8-byte alignment

        // timeToRun is a distribution of scheduling latencies, defined
        // as the sum of time a G spends in the _Grunnable state before
        // it transitions to _Grunning.
        //
        // timeToRun is protected by sched.lock.
        timeToRun timeHistogram
}

// Values for the flags field of a sigTabT.
const (
        _SigNotify   = 1 << iota // let signal.Notify have signal, even if from kernel
        _SigKill                 // if signal.Notify doesn't take it, exit quietly
        _SigThrow                // if signal.Notify doesn't take it, exit loudly
        _SigPanic                // if the signal is from the kernel, panic
        _SigDefault              // if the signal isn't explicitly requested, don't monitor it
        _SigGoExit               // cause all runtime procs to exit (only used on Plan 9).
        _SigSetStack             // add SA_ONSTACK to libc handler
        _SigUnblock              // always unblock; see blockableSig
        _SigIgn                  // _SIG_DFL action is to ignore the signal
)

// Layout of in-memory per-function information prepared by linker
// See https://golang.org/s/go12symtab.
// Keep in sync with linker (../cmd/link/internal/ld/pcln.go:/pclntab)
// and with package debug/gosym and with symtab.go in package runtime.
type _func struct {
        entryoff uint32 // start pc, as offset from moduledata.text/pcHeader.textStart
        nameoff  int32  // function name

        args        int32  // in/out args size
        deferreturn uint32 // offset of start of a deferreturn call instruction from entry, if any.

        pcsp      uint32
        pcfile    uint32
        pcln      uint32
        npcdata   uint32
        cuOffset  uint32 // runtime.cutab offset of this function's CU
        funcID    funcID // set for certain special runtime functions
        flag      funcFlag
        _         [1]byte // pad
        nfuncdata uint8   // must be last, must end on a uint32-aligned boundary
}

// Pseudo-Func that is returned for PCs that occur in inlined code.
// A *Func can be either a *_func or a *funcinl, and they are distinguished
// by the first uintptr.
type funcinl struct {
        ones  uint32  // set to ^0 to distinguish from _func
        entry uintptr // entry of the real (the "outermost") frame
        name  string
        file  string
        line  int
}

// layout of Itab known to compilers
// allocated in non-garbage-collected memory
// Needs to be in sync with
// ../cmd/compile/internal/reflectdata/reflect.go:/^func.WriteTabs.
type itab struct {
        inter *interfacetype
        _type *_type
        hash  uint32 // copy of _type.hash. Used for type switches.
        _     [4]byte
        fun   [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter.
}

// Lock-free stack node.
// Also known to export_test.go.
type lfnode struct {
        next    uint64
        pushcnt uintptr
}

type forcegcstate struct {
        lock mutex
        g    *g
        idle uint32
}

// extendRandom extends the random numbers in r[:n] to the whole slice r.
// Treats n<0 as n==0.
func extendRandom(r []byte, n int) {
        if n < 0 {
                n = 0
        }
        for n < len(r) {
                // Extend random bits using hash function & time seed
                w := n
                if w > 16 {
                        w = 16
                }
                h := memhash(unsafe.Pointer(&r[n-w]), uintptr(nanotime()), uintptr(w))
                for i := 0; i < goarch.PtrSize && n < len(r); i++ {
                        r[n] = byte(h)
                        n++
                        h >>= 8
                }
        }
}

// A _defer holds an entry on the list of deferred calls.
// If you add a field here, add code to clear it in freedefer and deferProcStack
// This struct must match the code in cmd/compile/internal/ssagen/ssa.go:deferstruct
// and cmd/compile/internal/ssagen/ssa.go:(*state).call.
// Some defers will be allocated on the stack and some on the heap.
// All defers are logically part of the stack, so write barriers to
// initialize them are not required. All defers must be manually scanned,
// and for heap defers, marked.
type _defer struct {
        started bool
        heap    bool
        // openDefer indicates that this _defer is for a frame with open-coded
        // defers. We have only one defer record for the entire frame (which may
        // currently have 0, 1, or more defers active).
        openDefer bool
        sp        uintptr // sp at time of defer
        pc        uintptr // pc at time of defer
        fn        func()  // can be nil for open-coded defers
        _panic    *_panic // panic that is running defer
        link      *_defer // next defer on G; can point to either heap or stack!

        // If openDefer is true, the fields below record values about the stack
        // frame and associated function that has the open-coded defer(s). sp
        // above will be the sp for the frame, and pc will be address of the
        // deferreturn call in the function.
        fd   unsafe.Pointer // funcdata for the function associated with the frame
        varp uintptr        // value of varp for the stack frame
        // framepc is the current pc associated with the stack frame. Together,
        // with sp above (which is the sp associated with the stack frame),
        // framepc/sp can be used as pc/sp pair to continue a stack trace via
        // gentraceback().
        framepc uintptr
}

// A _panic holds information about an active panic.
//
// A _panic value must only ever live on the stack.
//
// The argp and link fields are stack pointers, but don't need special
// handling during stack growth: because they are pointer-typed and
// _panic values only live on the stack, regular stack pointer
// adjustment takes care of them.
type _panic struct {
        argp      unsafe.Pointer // pointer to arguments of deferred call run during panic; cannot move - known to liblink
        arg       interface{}    // argument to panic
        link      *_panic        // link to earlier panic
        pc        uintptr        // where to return to in runtime if this panic is bypassed
        sp        unsafe.Pointer // where to return to in runtime if this panic is bypassed
        recovered bool           // whether this panic is over
        aborted   bool           // the panic was aborted
        goexit    bool
}

// stack traces
type stkframe struct {
        fn       funcInfo   // function being run
        pc       uintptr    // program counter within fn
        continpc uintptr    // program counter where execution can continue, or 0 if not
        lr       uintptr    // program counter at caller aka link register
        sp       uintptr    // stack pointer at pc
        fp       uintptr    // stack pointer at caller aka frame pointer
        varp     uintptr    // top of local variables
        argp     uintptr    // pointer to function arguments
        arglen   uintptr    // number of bytes at argp
        argmap   *bitvector // force use of this argmap
}

// ancestorInfo records details of where a goroutine was started.
type ancestorInfo struct {
        pcs  []uintptr // pcs from the stack of this goroutine
        goid int64     // goroutine id of this goroutine; original goroutine possibly dead
        gopc uintptr   // pc of go statement that created this goroutine
}

const (
        _TraceRuntimeFrames = 1 << iota // include frames for internal runtime functions.
        _TraceTrap                      // the initial PC, SP are from a trap, not a return PC from a call
        _TraceJumpStack                 // if traceback is on a systemstack, resume trace at g that called into it
)

// The maximum number of frames we print for a traceback
const _TracebackMaxFrames = 100

// A waitReason explains why a goroutine has been stopped.
// See gopark. Do not re-use waitReasons, add new ones.
type waitReason uint8

const (
        waitReasonZero                  waitReason = iota // ""
        waitReasonGCAssistMarking                         // "GC assist marking"
        waitReasonIOWait                                  // "IO wait"
        waitReasonChanReceiveNilChan                      // "chan receive (nil chan)"
        waitReasonChanSendNilChan                         // "chan send (nil chan)"
        waitReasonDumpingHeap                             // "dumping heap"
        waitReasonGarbageCollection                       // "garbage collection"
        waitReasonGarbageCollectionScan                   // "garbage collection scan"
        waitReasonPanicWait                               // "panicwait"
        waitReasonSelect                                  // "select"
        waitReasonSelectNoCases                           // "select (no cases)"
        waitReasonGCAssistWait                            // "GC assist wait"
        waitReasonGCSweepWait                             // "GC sweep wait"
        waitReasonGCScavengeWait                          // "GC scavenge wait"
        waitReasonChanReceive                             // "chan receive"
        waitReasonChanSend                                // "chan send"
        waitReasonFinalizerWait                           // "finalizer wait"
        waitReasonForceGCIdle                             // "force gc (idle)"
        waitReasonSemacquire                              // "semacquire"
        waitReasonSleep                                   // "sleep"
        waitReasonSyncCondWait                            // "sync.Cond.Wait"
        waitReasonTimerGoroutineIdle                      // "timer goroutine (idle)"
        waitReasonTraceReaderBlocked                      // "trace reader (blocked)"
        waitReasonWaitForGCCycle                          // "wait for GC cycle"
        waitReasonGCWorkerIdle                            // "GC worker (idle)"
        waitReasonPreempted                               // "preempted"
        waitReasonDebugCall                               // "debug call"
)

var waitReasonStrings = [...]string{
        waitReasonZero:                  "",
        waitReasonGCAssistMarking:       "GC assist marking",
        waitReasonIOWait:                "IO wait",
        waitReasonChanReceiveNilChan:    "chan receive (nil chan)",
        waitReasonChanSendNilChan:       "chan send (nil chan)",
        waitReasonDumpingHeap:           "dumping heap",
        waitReasonGarbageCollection:     "garbage collection",
        waitReasonGarbageCollectionScan: "garbage collection scan",
        waitReasonPanicWait:             "panicwait",
        waitReasonSelect:                "select",
        waitReasonSelectNoCases:         "select (no cases)",
        waitReasonGCAssistWait:          "GC assist wait",
        waitReasonGCSweepWait:           "GC sweep wait",
        waitReasonGCScavengeWait:        "GC scavenge wait",
        waitReasonChanReceive:           "chan receive",
        waitReasonChanSend:              "chan send",
        waitReasonFinalizerWait:         "finalizer wait",
        waitReasonForceGCIdle:           "force gc (idle)",
        waitReasonSemacquire:            "semacquire",
        waitReasonSleep:                 "sleep",
        waitReasonSyncCondWait:          "sync.Cond.Wait",
        waitReasonTimerGoroutineIdle:    "timer goroutine (idle)",
        waitReasonTraceReaderBlocked:    "trace reader (blocked)",
        waitReasonWaitForGCCycle:        "wait for GC cycle",
        waitReasonGCWorkerIdle:          "GC worker (idle)",
        waitReasonPreempted:             "preempted",
        waitReasonDebugCall:             "debug call",
}

func (w waitReason) String() string {
        if w < 0 || w >= waitReason(len(waitReasonStrings)) {
                return "unknown wait reason"
        }
        return waitReasonStrings[w]
}

var (
        allm       *m
        gomaxprocs int32
        ncpu       int32
        forcegc    forcegcstate
        sched      schedt
        newprocs   int32

        // allpLock protects P-less reads and size changes of allp, idlepMask,
        // and timerpMask, and all writes to allp.
        allpLock mutex
        // len(allp) == gomaxprocs; may change at safe points, otherwise
        // immutable.
        allp []*p
        // Bitmask of Ps in _Pidle list, one bit per P. Reads and writes must
        // be atomic. Length may change at safe points.
        //
        // Each P must update only its own bit. In order to maintain
        // consistency, a P going idle must the idle mask simultaneously with
        // updates to the idle P list under the sched.lock, otherwise a racing
        // pidleget may clear the mask before pidleput sets the mask,
        // corrupting the bitmap.
        //
        // N.B., procresize takes ownership of all Ps in stopTheWorldWithSema.
        idlepMask pMask
        // Bitmask of Ps that may have a timer, one bit per P. Reads and writes
        // must be atomic. Length may change at safe points.
        timerpMask pMask

        // Pool of GC parked background workers. Entries are type
        // *gcBgMarkWorkerNode.
        gcBgMarkWorkerPool lfstack

        // Total number of gcBgMarkWorker goroutines. Protected by worldsema.
        gcBgMarkWorkerCount int32

        // Information about what cpu features are available.
        // Packages outside the runtime should not use these
        // as they are not an external api.
        // Set on startup in asm_{386,amd64}.s
        processorVersionInfo uint32
        isIntel              bool

        goarm uint8 // set by cmd/link on arm systems
)

// Set by the linker so the runtime can determine the buildmode.
var (
        islibrary bool // -buildmode=c-shared
        isarchive bool // -buildmode=c-archive
)

// Must agree with internal/buildcfg.Experiment.FramePointer.
const framepointer_enabled = GOARCH == "amd64" || GOARCH == "arm64"