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runtime: synchronize P wakeup and dropping Ps

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CL 310850 dropped work re-checks on non-spinning Ms to fix #43997.

This introduced a new race condition: a non-spinning M may drop its P
and then park at the same time a spinning M attempts to wake a P to
handle some new work. The spinning M fails to find an idle P (because
the non-spinning M hasn't quite made its P idle yet), and does nothing
assuming that the system is fully loaded. This results in loss of work
conservation. In the worst case we could have a complete deadlock if
injectglist fails to wake anything just as all Ps are going idle.

sched.needspinning adds new synchronization to cover this case. If work
submission fails to find a P, it sets needspinning to indicate that a
spinning M is required. When non-spinning Ms prepare to drop their P,
they check needspinning and abort going idle to become a spinning M
instead. This addresses the race without extra spurious wakeups. In the
normal (non-racing case), an M will become spinning via the normal path
and clear the flag.

injectglist must change in addition to wakep because it is a similar
form of work submission, notably used following netpoll at a point when
we might not have a P that would guarantee the work runs.

Fixes #45867

Change-Id: Ieb623a6d4162fb8c2be7b4ff8acdebcc3a0d69a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/389014
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
This commit is contained in:
Michael Pratt 2022-03-01 15:06:37 -05:00 • committed by Gopher Robot
parent 54cf1b107d
commit 8cb350d69a
2 changed files with 140 additions and 46 deletions
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9
src/runtime/runtime2.go
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@ -777,9 +777,10 @@ type schedt struct {
ngsys atomic.Int32 // number of system goroutines
pidle puintptr // idle p's
npidle atomic.Int32
nmspinning atomic.Int32 // See "Worker thread parking/unparking" comment in proc.go.
pidle puintptr // idle p's
npidle atomic.Int32
nmspinning atomic.Int32 // See "Worker thread parking/unparking" comment in proc.go.
needspinning atomic.Uint32 // See "Delicate dance" comment in proc.go. Boolean. Must hold sched.lock to set to 1.
// Global runnable queue.
runq gQueue
@ -840,6 +841,8 @@ type schedt struct {
// with the rest of the runtime.
sysmonlock mutex
_ uint32 // ensure timeToRun has 8-byte alignment
// timeToRun is a distribution of scheduling latencies, defined
// as the sum of time a G spends in the _Grunnable state before
// it transitions to _Grunning.