runtime: detect netbsd netpoll overrun in sysmon The netbsd kernel has a bug [1] that occassionally prevents netpoll from waking with netpollBreak, which could result in missing timers for an unbounded amount of time, as netpoll can't restart with a shorter delay when an earlier timer is added. Prior to CL 232298, sysmon could detect these overrun timers and manually start an M to run them. With this fallback gone, the bug actually prevents timer execution indefinitely. As a workaround, we add back sysmon detection only for netbsd. [1] https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=50094 Updates #42515 Change-Id: I8391f5b9dabef03dd1d94c50b3b4b3bd4f889e66 Reviewed-on: https://go-review.googlesource.com/c/go/+/277332 Run-TryBot: Michael Pratt <mpratt@google.com> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Austin Clements <austin@google.com> Trust: Michael Pratt <mpratt@google.com>
diff --git a/src/runtime/proc.go b/src/runtime/proc.go index 418e069..5adcbf0 100644 --- a/src/runtime/proc.go +++ b/src/runtime/proc.go
@@ -5130,6 +5130,26 @@ } } mDoFixup() + if GOOS == "netbsd" { + // netpoll is responsible for waiting for timer + // expiration, so we typically don't have to worry + // about starting an M to service timers. (Note that + // sleep for timeSleepUntil above simply ensures sysmon + // starts running again when that timer expiration may + // cause Go code to run again). + // + // However, netbsd has a kernel bug that sometimes + // misses netpollBreak wake-ups, which can lead to + // unbounded delays servicing timers. If we detect this + // overrun, then startm to get something to handle the + // timer. + // + // See issue 42515 and + // https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=50094. + if next, _ := timeSleepUntil(); next < now { + startm(nil, false) + } + } if atomic.Load(&scavenge.sysmonWake) != 0 { // Kick the scavenger awake if someone requested it. wakeScavenger()