runtime: fix leak and locking in BenchmarkMSpanCountAlloc

CL 249917 made the mspan in MSpanCountAlloc no longer stack-allocated
(for good reason), but then allocated an mspan on each call and did not
free it, resulting in a leak. That allocation was also not protected by
the heap lock, which could lead to data corruption of mheap fields and
the spanalloc.

To fix this, export some functions to allocate/free dummy mspans from
spanalloc (with proper locking) and allocate just one up-front for the
benchmark, freeing it at the end. Then, update MSpanCountAlloc to accept
a dummy mspan.

Note that we need to allocate the dummy mspan up-front otherwise we
measure things like heap locking and fixalloc performance instead of
what we actually want to measure: how fast we can do a popcount on the
mark bits.

Fixes #41391.

Change-Id: If6629a6ec1ece639c7fb78532045837a8c872c04
Reviewed-on: https://go-review.googlesource.com/c/go/+/255297
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/src/runtime/gc_test.go b/src/runtime/gc_test.go
index c5c8a4cecf..9edebdada6 100644
--- a/src/runtime/gc_test.go
+++ b/src/runtime/gc_test.go
@@ -763,6 +763,10 @@ func BenchmarkScanStackNoLocals(b *testing.B) {
 }
 
 func BenchmarkMSpanCountAlloc(b *testing.B) {
+	// Allocate one dummy mspan for the whole benchmark.
+	s := runtime.AllocMSpan()
+	defer runtime.FreeMSpan(s)
+
 	// n is the number of bytes to benchmark against.
 	// n must always be a multiple of 8, since gcBits is
 	// always rounded up 8 bytes.
@@ -774,7 +778,7 @@ func BenchmarkMSpanCountAlloc(b *testing.B) {
 
 			b.ResetTimer()
 			for i := 0; i < b.N; i++ {
-				runtime.MSpanCountAlloc(bits)
+				runtime.MSpanCountAlloc(s, bits)
 			}
 		})
 	}