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runtime: abstract out assist queue management

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This puts all of the assist queue-related code together and makes it
easier to modify how the assist queue works.

Change-Id: Id54e06702bdd5a5dd3fef2ce2c14cd7ca215303c
Reviewed-on: https://go-review.googlesource.com/30700
Reviewed-by: Rick Hudson <rlh@golang.org>
This commit is contained in:
Austin Clements 2016-10-06 15:12:12 -04:00
parent 395d36a67d
commit 81c431a537
1 changed files with 45 additions and 32 deletions
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77
src/runtime/mgcmark.go
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@ -538,40 +538,9 @@ retry:
// there wasn't enough work to do anyway, so we might
// as well let background marking take care of the
// work that is available.
lock(&work.assistQueue.lock)
// If the GC cycle is over, just return. This is the
// likely path if we completed above. We do this
// under the lock to prevent a GC cycle from ending
// between this check and queuing the assist.
if atomic.Load(&gcBlackenEnabled) == 0 {
unlock(&work.assistQueue.lock)
return
}
oldHead, oldTail := work.assistQueue.head, work.assistQueue.tail
if oldHead == 0 {
work.assistQueue.head.set(gp)
} else {
oldTail.ptr().schedlink.set(gp)
}
work.assistQueue.tail.set(gp)
gp.schedlink.set(nil)
// Recheck for background credit now that this G is in
// the queue, but can still back out. This avoids a
// race in case background marking has flushed more
// credit since we checked above.
if atomic.Loadint64(&gcController.bgScanCredit) > 0 {
work.assistQueue.head = oldHead
work.assistQueue.tail = oldTail
if oldTail != 0 {
oldTail.ptr().schedlink.set(nil)
}
unlock(&work.assistQueue.lock)
if !gcParkAssist() {
goto retry
}
// Park for real.
goparkunlock(&work.assistQueue.lock, "GC assist wait", traceEvGoBlock, 2)
// At this point either background GC has satisfied
// this G's assist debt, or the GC cycle is over.
@ -589,6 +558,50 @@ func gcWakeAllAssists() {
unlock(&work.assistQueue.lock)
}
// gcParkAssist puts the current goroutine on the assist queue and parks.
//
// gcParkAssist returns whether the assist is now satisfied. If it
// returns false, the caller must retry the assist.
//
//go:nowritebarrier
func gcParkAssist() bool {
lock(&work.assistQueue.lock)
// If the GC cycle finished while we were getting the lock,
// exit the assist. The cycle can't finish while we hold the
// lock.
if atomic.Load(&gcBlackenEnabled) == 0 {
unlock(&work.assistQueue.lock)
return true
}
gp := getg()
oldHead, oldTail := work.assistQueue.head, work.assistQueue.tail
if oldHead == 0 {
work.assistQueue.head.set(gp)
} else {
oldTail.ptr().schedlink.set(gp)
}
work.assistQueue.tail.set(gp)
gp.schedlink.set(nil)
// Recheck for background credit now that this G is in
// the queue, but can still back out. This avoids a
// race in case background marking has flushed more
// credit since we checked above.
if atomic.Loadint64(&gcController.bgScanCredit) > 0 {
work.assistQueue.head = oldHead
work.assistQueue.tail = oldTail
if oldTail != 0 {
oldTail.ptr().schedlink.set(nil)
}
unlock(&work.assistQueue.lock)
return false
}
// Park.
goparkunlock(&work.assistQueue.lock, "GC assist wait", traceEvGoBlock, 2)
return true
}
// gcFlushBgCredit flushes scanWork units of background scan work
// credit. This first satisfies blocked assists on the
// work.assistQueue and then flushes any remaining credit to