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://golang.org/cl/12176043

diff --git a/src/pkg/runtime/proc.c b/src/pkg/runtime/proc.c
index c4b8c025173b26a3dfdfa3758a8ca4328f336148..c3af1efaf309197a86bb50080da55b1b943ee705 100644 (file)
--- a/src/pkg/runtime/proc.c
+++ b/src/pkg/runtime/proc.c
@@ -1536,6 +1536,10 @@ exitsyscallfast(void)
        if(runtime·sched.pidle) {
                runtime·lock(&runtime·sched);
                p = pidleget();
+               if(p && runtime·atomicload(&runtime·sched.sysmonwait)) {
+                       runtime·atomicstore(&runtime·sched.sysmonwait, 0);
+                       runtime·notewakeup(&runtime·sched.sysmonnote);
+               }
                runtime·unlock(&runtime·sched);
                if(p) {
                        acquirep(p);
@@ -1559,6 +1563,10 @@ exitsyscall0(G *gp)
        p = pidleget();
        if(p == nil)
                globrunqput(gp);
+       else if(runtime·atomicload(&runtime·sched.sysmonwait)) {
+               runtime·atomicstore(&runtime·sched.sysmonwait, 0);
+               runtime·notewakeup(&runtime·sched.sysmonnote);
+       }
        runtime·unlock(&runtime·sched);
        if(p) {
                acquirep(p);