runtime: apply looser bound to /gc/heap/live:bytes in test

/gc/heap/live:bytes may exceed MemStats.HeapAlloc, even when all data is
flushed, becuase the GC may double-count objects when marking them. This
is an intentional design choice that is largely inconsequential. The
runtime is already robust to it, and the condition is rare.

Fixes #60607.

Change-Id: I4da402efc24327328d2d8780e4e49961b189f0ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/501858
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

diff --git a/src/runtime/metrics_test.go b/src/runtime/metrics_test.go
index c138a2a1ba0d448b6316377fe76b5b27dccb475c..a64e898739731a500e135392e362f098b512f843 100644 (file)
--- a/src/runtime/metrics_test.go
+++ b/src/runtime/metrics_test.go
@@ -143,8 +143,16 @@ func TestReadMetrics(t *testing.T) {
                case "/gc/heap/frees:objects":
                        frees = samples[i].Value.Uint64()
                case "/gc/heap/live:bytes":
-                       if live := samples[i].Value.Uint64(); live > mstats.HeapAlloc {
-                               t.Errorf("live bytes: %d > heap alloc: %d", live, mstats.HeapAlloc)
+                       // Check for "obviously wrong" values. We can't check a stronger invariant,
+                       // such as live <= HeapAlloc, because live is not 100% accurate. It's computed
+                       // under racy conditions, and some objects may be double-counted (this is
+                       // intentional and necessary for GC performance).
+                       //
+                       // Instead, check against a much more reasonable upper-bound: the amount of
+                       // mapped heap memory. We can't possibly overcount to the point of exceeding
+                       // total mapped heap memory, except if there's an accounting bug.
+                       if live := samples[i].Value.Uint64(); live > mstats.HeapSys {
+                               t.Errorf("live bytes: %d > heap sys: %d", live, mstats.HeapSys)
                        } else if live == 0 {
                                // Might happen if we don't call runtime.GC() above.
                                t.Error("live bytes is 0")