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runtime: work around "P has cached GC work" failures

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We still don't understand what's causing there to be remaining GC work
when we enter mark termination, but in order to move forward on this
issue, this CL implements a work-around for the problem.

If debugCachedWork is false, this CL does a second check for remaining
GC work as soon as it stops the world for mark termination. If it
finds any work, it starts the world again and re-enters concurrent
mark. This will increase STW time by a small amount proportional to
GOMAXPROCS, but fixes a serious correctness issue.

This works-around #27993.

Change-Id: Ia23b85dd6c792ee8d623428bd1a3115631e387b8
Reviewed-on: https://go-review.googlesource.com/c/156140
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>
This commit is contained in:
Austin Clements 2019-01-03 14:48:30 -05:00
parent 9a7278ae47
commit 95a6f112c6
1 changed files with 28 additions and 1 deletions
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29
src/runtime/mgc.go
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@ -1367,7 +1367,7 @@ var gcMarkDoneFlushed uint32
// termination.
//
// For debugging issue #27993.
const debugCachedWork = true
const debugCachedWork = false
// gcWorkPauseGen is for debugging the mark completion algorithm.
// gcWork put operations spin while gcWork.pauseGen == gcWorkPauseGen.
@ -1525,6 +1525,33 @@ top:
throw("throwOnGCWork")
}
}
} else {
// For unknown reasons (see issue #27993), there is
// sometimes work left over when we enter mark
// termination. Detect this and resume concurrent
// mark. This is obviously unfortunate.
//
// Switch to the system stack to call wbBufFlush1,
// though in this case it doesn't matter because we're
// non-preemptible anyway.
restart := false
systemstack(func() {
for _, p := range allp {
wbBufFlush1(p)
if !p.gcw.empty() {
restart = true
break
}
}
})
if restart {
getg().m.preemptoff = ""
systemstack(func() {
now := startTheWorldWithSema(true)
work.pauseNS += now - work.pauseStart
})
goto top
}
}
// Disable assists and background workers. We must do