From ad7a6f81597fae99f2d94dda308bdc19ff8c1362 Mon Sep 17 00:00:00 2001
From: Michael Anthony Knyszek <mknyszek@google.com>
Date: Wed, 7 May 2025 22:28:23 +0000
Subject: [PATCH] runtime: fix condition to emit gcpacertrace end-of-sweep line
It's the job of the last sweeper to emit the GC pacer trace. The last
sweeper can identify themselves by reducing the count of sweepers, and
also seeing that there's no more sweep work.
Currently this identification is broken, however, because the last
sweeper doesn't check the state they just transitioned sweeping into,
but rather the state they transitioned from (one sweeper, no sweep work
left). By design, it's impossible to transition *out* of this state,
except for another GC to start, but that doesn't take this codepath.
This means lines like
pacer: sweep done at heap size ...
were missing from the gcpacertrace output for a long time.
This change fixes this problem by having the last sweeper check the
state they just transitioned sweeping to, instead of the state they
transitioned from.
Change-Id: I44bcd32fe2c8ae6ac6c21ba6feb2e7b9e17f60cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/670735
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
---
src/runtime/mgcsweep.go | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/src/runtime/mgcsweep.go b/src/runtime/mgcsweep.go
index 1a9c3b3e5f9..191935dfd57 100644
--- a/src/runtime/mgcsweep.go
+++ b/src/runtime/mgcsweep.go
@@ -169,9 +169,10 @@ func (a *activeSweep) end(sl sweepLocker) {
throw("mismatched begin/end of activeSweep")
}
if a.state.CompareAndSwap(state, state-1) {
- if state != sweepDrainedMask {
+ if state-1 != sweepDrainedMask {
return
}
+ // We're the last sweeper, and there's nothing left to sweep.
if debug.gcpacertrace > 0 {
live := gcController.heapLive.Load()
print("pacer: sweep done at heap size ", live>>20, "MB; allocated ", (live-mheap_.sweepHeapLiveBasis)>>20, "MB during sweep; swept ", mheap_.pagesSwept.Load(), " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n")
--
GitLab