runtime: start an M when handing off a P when there's GC work
Currently it's possible for the scheduler to deadlock with the right
confluence of locked Gs, assists, and scheduling of background mark
workers. Broadly, this happens because handoffp is stricter than
findrunnable, and if the only work for a P is GC work, handoffp will
put the P into idle, rather than starting an M to execute that P. One
way this can happen is as follows:
0. There is only one user G, which we'll call G 1. There is more than
one P, but they're all idle except the one running G 1.
1. G 1 locks itself to an M using runtime.LockOSThread.
2. GC starts up and enters mark 1.
3. G 1 performs a GC assist, which completes mark 1 without being
fully satisfied. Completing mark 1 causes all background mark
workers to park. And since the assist isn't fully satisfied, it
parks as well, waiting for a background mark worker to satisfy its
remaining assist debt.
4. The assist park enters the scheduler. Since G 1 is locked to the M,
the scheduler releases the P and calls handoffp to hand the P to
another M.
5. handoffp checks the local and global run queues, which are empty,
and sees that there are idle Ps, so rather than start an M, it puts
the P into idle.
At this point, all of the Gs are waiting and all of the Ps are idle.
In particular, none of the GC workers are running, so no mark work
gets done and the assist on the main G is never satisfied, so the
whole process soft locks up.
Fix this by making handoffp start an M if there is GC work. This
reintroduces a key invariant: that in any situation where findrunnable
would return a G to run on a P, handoffp for that P will start an M to
run work on that P.
Fixes #13645.
Tested by running 2,689 iterations of `go tool dist test -no-rebuild
runtime:cpu124` across 10 linux-amd64-noopt VMs with no failures.
Without this change, the failure rate was somewhere around 1%.
Performance change is negligible.
name old time/op new time/op delta
XBenchGarbage-12 2.48ms ± 2% 2.48ms ± 1% -0.24% (p=0.000 n=92+93)
name old time/op new time/op delta
BinaryTree17-12 2.86s ± 2% 2.87s ± 2% ~ (p=0.667 n=19+20)
Fannkuch11-12 2.52s ± 1% 2.47s ± 1% -2.05% (p=0.000 n=18+20)
FmtFprintfEmpty-12 51.7ns ± 1% 51.5ns ± 3% ~ (p=0.931 n=16+20)
FmtFprintfString-12 170ns ± 1% 168ns ± 1% -0.65% (p=0.000 n=19+19)
FmtFprintfInt-12 160ns ± 0% 160ns ± 0% +0.18% (p=0.033 n=17+19)
FmtFprintfIntInt-12 265ns ± 1% 273ns ± 1% +2.98% (p=0.000 n=17+19)
FmtFprintfPrefixedInt-12 235ns ± 1% 239ns ± 1% +1.99% (p=0.000 n=16+19)
FmtFprintfFloat-12 315ns ± 0% 315ns ± 1% ~ (p=0.250 n=17+19)
FmtManyArgs-12 1.04µs ± 1% 1.05µs ± 0% +0.87% (p=0.000 n=17+19)
GobDecode-12 7.93ms ± 0% 7.85ms ± 1% -1.03% (p=0.000 n=16+18)
GobEncode-12 6.62ms ± 1% 6.58ms ± 1% -0.60% (p=0.000 n=18+19)
Gzip-12 322ms ± 1% 320ms ± 1% -0.46% (p=0.009 n=20+20)
Gunzip-12 42.5ms ± 1% 42.5ms ± 0% ~ (p=0.751 n=19+19)
HTTPClientServer-12 69.7µs ± 1% 70.0µs ± 2% ~ (p=0.056 n=19+19)
JSONEncode-12 16.9ms ± 1% 16.7ms ± 1% -1.13% (p=0.000 n=19+19)
JSONDecode-12 61.5ms ± 1% 61.3ms ± 1% -0.35% (p=0.001 n=20+17)
Mandelbrot200-12 3.94ms ± 0% 3.91ms ± 0% -0.67% (p=0.000 n=20+18)
GoParse-12 3.71ms ± 1% 3.70ms ± 1% ~ (p=0.244 n=17+19)
RegexpMatchEasy0_32-12 101ns ± 1% 102ns ± 2% +0.54% (p=0.037 n=19+20)
RegexpMatchEasy0_1K-12 349ns ± 0% 350ns ± 0% +0.33% (p=0.000 n=17+18)
RegexpMatchEasy1_32-12 84.5ns ± 2% 84.2ns ± 1% -0.43% (p=0.048 n=19+20)
RegexpMatchEasy1_1K-12 510ns ± 1% 513ns ± 2% +0.58% (p=0.002 n=18+20)
RegexpMatchMedium_32-12 132ns ± 1% 134ns ± 1% +0.95% (p=0.000 n=20+20)
RegexpMatchMedium_1K-12 40.1µs ± 1% 39.6µs ± 1% -1.39% (p=0.000 n=20+20)
RegexpMatchHard_32-12 2.08µs ± 0% 2.06µs ± 1% -0.95% (p=0.000 n=18+18)
RegexpMatchHard_1K-12 62.2µs ± 1% 61.9µs ± 1% -0.42% (p=0.001 n=19+20)
Revcomp-12 537ms ± 0% 536ms ± 0% ~ (p=0.076 n=20+20)
Template-12 71.3ms ± 1% 69.3ms ± 1% -2.75% (p=0.000 n=20+20)
TimeParse-12 361ns ± 0% 360ns ± 1% ~ (p=0.056 n=19+19)
TimeFormat-12 353ns ± 0% 352ns ± 0% -0.23% (p=0.000 n=17+18)
[Geo mean] 62.6µs 62.5µs -0.17%
Change-Id: I0fbbbe4d7d99653ba5600ffb4394fa03558bc4e9
Reviewed-on: https://go-review.googlesource.com/19107
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 2bc3c92..d1f5088 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -1602,11 +1602,19 @@
// Always runs without a P, so write barriers are not allowed.
//go:nowritebarrier
func handoffp(_p_ *p) {
+ // handoffp must start an M in any situation where
+ // findrunnable would return a G to run on _p_.
+
// if it has local work, start it straight away
if !runqempty(_p_) || sched.runqsize != 0 {
startm(_p_, false)
return
}
+ // if it has GC work, start it straight away
+ if gcBlackenEnabled != 0 && gcMarkWorkAvailable(_p_) {
+ startm(_p_, false)
+ return
+ }
// no local work, check that there are no spinning/idle M's,
// otherwise our help is not required
if atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) == 0 && atomic.Cas(&sched.nmspinning, 0, 1) { // TODO: fast atomic
@@ -1787,6 +1795,10 @@
func findrunnable() (gp *g, inheritTime bool) {
_g_ := getg()
+ // The conditions here and in handoffp must agree: if
+ // findrunnable would return a G to run, handoffp must start
+ // an M.
+
top:
if sched.gcwaiting != 0 {
gcstopm()
