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runtime: preempt dedicated background mark workers for STW

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Currently, dedicated background mark workers are essentially always
non-preemptible.

This change makes it so that dedicated background mark workers park if
their preemption flag is set and someone is trying to STW, allowing them
to do so.

This change prepares us for allowing a STW to happen (and happen
promptly) during GC marking in a follow-up change.

Updates #19812.

Change-Id: I67fb6085bf0f0aebd18ca500172767818a1f15e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/215157
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
This commit is contained in:
Michael Anthony Knyszek 2020-01-16 21:39:42 +00:00 committed by Michael Knyszek
parent 6197104c14
commit 79b43fa819
1 changed files with 6 additions and 2 deletions
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8
src/runtime/mgcmark.go
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@ -963,6 +963,8 @@ const (
// credit to gcController.bgScanCredit every gcCreditSlack units of
// scan work.
//
// gcDrain will always return if there is a pending STW.
//
//go:nowritebarrier
func gcDrain(gcw *gcWork, flags gcDrainFlags) {
if !writeBarrier.needed {
@ -991,7 +993,8 @@ func gcDrain(gcw *gcWork, flags gcDrainFlags) {
// Drain root marking jobs.
if work.markrootNext < work.markrootJobs {
for !(preemptible && gp.preempt) {
// Stop if we're preemptible or if someone wants to STW.
for !(gp.preempt && (preemptible || atomic.Load(&sched.gcwaiting) != 0)) {
job := atomic.Xadd(&work.markrootNext, +1) - 1
if job >= work.markrootJobs {
break
@ -1004,7 +1007,8 @@ func gcDrain(gcw *gcWork, flags gcDrainFlags) {
}
// Drain heap marking jobs.
for !(preemptible && gp.preempt) {
// Stop if we're preemptible or if someone wants to STW.
for !(gp.preempt && (preemptible || atomic.Load(&sched.gcwaiting) != 0)) {
// Try to keep work available on the global queue. We used to
// check if there were waiting workers, but it's better to
// just keep work available than to make workers wait. In the