From 79b43fa819bca31b5be2b9ed3014d8b7faf4c8cc Mon Sep 17 00:00:00 2001 From: Michael Anthony Knyszek Date: Thu, 16 Jan 2020 21:39:42 +0000 Subject: [PATCH] runtime: preempt dedicated background mark workers for STW 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 TryBot-Result: Gobot Gobot Reviewed-by: Cherry Zhang Reviewed-by: Austin Clements --- src/runtime/mgcmark.go | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go index 54f988a902..2c17d8befa 100644 --- a/src/runtime/mgcmark.go +++ b/src/runtime/mgcmark.go @@ -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