runtime: factor checking if P run queue is empty

There are a variety of places where we check if a P's run queue is
empty. This test is about to get slightly more complicated, so factor
it out into a new function, runqempty. This function is inlinable, so
this has no effect on performance.

Change-Id: If4a0b01ffbd004937de90d8d686f6ded4aad2c6b
Reviewed-on: https://go-review.googlesource.com/9287
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

diff --git a/src/runtime/proc1.go b/src/runtime/proc1.go
index 0359b5571c484749236d6925e9f20109e9e7c3fd..d37c4f1a5acd6341ef1e0b1bf517f6659f021b85 100644 (file)
--- a/src/runtime/proc1.go
+++ b/src/runtime/proc1.go
@@ -1115,7 +1115,7 @@ func startm(_p_ *p, spinning bool) {
 //go:nowritebarrier
 func handoffp(_p_ *p) {
        // if it has local work, start it straight away
-       if _p_.runqhead != _p_.runqtail || sched.runqsize != 0 {
+       if !runqempty(_p_) || sched.runqsize != 0 {
                startm(_p_, false)
                return
        }
@@ -1369,7 +1369,7 @@ stop:
        // check all runqueues once again
        for i := 0; i < int(gomaxprocs); i++ {
                _p_ := allp[i]
-               if _p_ != nil && _p_.runqhead != _p_.runqtail {
+               if _p_ != nil && !runqempty(_p_) {
                        lock(&sched.lock)
                        _p_ = pidleget()
                        unlock(&sched.lock)
@@ -2657,7 +2657,7 @@ func procresize(nprocs int32) *p {
                        continue
                }
                p.status = _Pidle
-               if p.runqhead == p.runqtail {
+               if runqempty(p) {
                        pidleput(p)
                } else {
                        p.m.set(mget())
@@ -2940,7 +2940,7 @@ func retake(now int64) uint32 {
                        // On the one hand we don't want to retake Ps if there is no other work to do,
                        // but on the other hand we want to retake them eventually
                        // because they can prevent the sysmon thread from deep sleep.
-                       if _p_.runqhead == _p_.runqtail && atomicload(&sched.nmspinning)+atomicload(&sched.npidle) > 0 && pd.syscallwhen+10*1000*1000 > now {
+                       if runqempty(_p_) && atomicload(&sched.nmspinning)+atomicload(&sched.npidle) > 0 && pd.syscallwhen+10*1000*1000 > now {
                                continue
                        }
                        // Need to decrement number of idle locked M's
@@ -3220,6 +3220,12 @@ func pidleget() *p {
        return _p_
 }
 
+// runqempty returns true if _p_ has no Gs on its local run queue.
+// Note that this test is generally racy.
+func runqempty(_p_ *p) bool {
+       return _p_.runqhead == _p_.runqtail
+}
+
 // Try to put g on local runnable queue.
 // If it's full, put onto global queue.
 // Executed only by the owner P.
@@ -3479,7 +3485,7 @@ func sync_runtime_canSpin(i int) bool {
        if i >= active_spin || ncpu <= 1 || gomaxprocs <= int32(sched.npidle+sched.nmspinning)+1 {
                return false
        }
-       if p := getg().m.p.ptr(); p.runqhead != p.runqtail {
+       if p := getg().m.p.ptr(); !runqempty(p) {
                return false
        }
        return true