code review 12176043: runtime: do not park sysmon thread if any goroutines ar...

runtime: do not park sysmon thread if any goroutines are running
Sysmon thread parks if no goroutines are running (runtime.sched.npidle ==
runtime.gomaxprocs).
Currently it's unparked when a goroutine enters syscall, it was enough
to retake P's from blocking syscalls.
But it's not enough for reliable goroutine preemption. We need to ensure that
sysmon runs if any goroutines are running.

[dvyukov, 2013-07-31 16:08:25]

Hello rsc@golang.org (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://dvyukov%40google.com@code.google.com/p/go/

[dvyukov, 2013-07-31 16:08:50]

This is the same as https://codereview.appspot.com/12167043/
but w/o unrelated changes this time.

[dvyukov, 2013-07-31 16:09:14]

*** Submitted as https://code.google.com/p/go/source/detail?r=aaec807a07a1 ***

runtime: do not park sysmon thread if any goroutines are running
Sysmon thread parks if no goroutines are running (runtime.sched.npidle ==
runtime.gomaxprocs).
Currently it's unparked when a goroutine enters syscall, it was enough
to retake P's from blocking syscalls.
But it's not enough for reliable goroutine preemption. We need to ensure that
sysmon runs if any goroutines are running.

R=rsc
CC=golang-dev
https://codereview.appspot.com/12176043

[rsc, 2013-08-02 15:54:44]

LGTM