runtime: simplify forced GC triggering

Now that the gcMode is no longer involved in the GC trigger condition,
we can simplify the triggering of forced GCs. By making the trigger
condition for forced GCs true even if gcphase is not _GCoff, we don't
need any special case path in gcStart to ensure that forced GCs don't
get consolidated.

Change-Id: I6067a13d76e40ff2eef8fade6fc14adb0cb58ee5
Reviewed-on: https://go-review.googlesource.com/37517
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -911,8 +911,8 @@ type gcTriggerKind int
 
 const (
 	// gcTriggerAlways indicates that a cycle should be started
-	// unconditionally, even if GOGC is off. This cannot be
-	// consolidated with other cycles.
+	// unconditionally, even if GOGC is off or we're in a cycle
+	// right now. This cannot be consolidated with other cycles.
 	gcTriggerAlways gcTriggerKind = iota
 
 	// gcTriggerHeap indicates that a cycle should be started when
@@ -930,13 +930,13 @@ const (
 // that the exit condition for the _GCoff phase has been met. The exit
 // condition should be tested when allocating.
 func (t gcTrigger) test() bool {
-	if !(gcphase == _GCoff && memstats.enablegc && panicking == 0) {
+	if !memstats.enablegc || panicking != 0 {
 		return false
 	}
 	if t.kind == gcTriggerAlways {
 		return true
 	}
-	if gcpercent < 0 {
+	if gcphase != _GCoff || gcpercent < 0 {
 		return false
 	}
 	switch t.kind {
@@ -984,20 +984,12 @@ func gcStart(mode gcMode, trigger gcTrigger) {
 
 	// Perform GC initialization and the sweep termination
 	// transition.
-	//
-	// If this is a forced GC, don't acquire the transition lock
-	// or re-check the transition condition because we
-	// specifically *don't* want to share the transition with
-	// another thread.
-	useStartSema := trigger.kind != gcTriggerAlways
-	if useStartSema {
 	semacquire(&work.startSema)
 	// Re-check transition condition under transition lock.
 	if !trigger.test() {
 		semrelease(&work.startSema)
 		return
 	}
-	}
 
 	// For stats, check if this GC was forced by the user.
 	forced := trigger.kind == gcTriggerAlways
@@ -1103,9 +1095,7 @@ func gcStart(mode gcMode, trigger gcTrigger) {
 		gcMarkTermination()
 	}
 
-	if useStartSema {
 	semrelease(&work.startSema)
-	}
 }
 
 // gcMarkDone transitions the GC from mark 1 to mark 2 and from mark 2