runtime: generalize GC trigger

Currently the GC triggering condition is an awkward combination of the
gcMode (whether or not it's gcBackgroundMode) and a boolean
"forceTrigger" flag.

Replace this with a new gcTrigger type that represents the range of
transition predicates we need. This has several advantages:

1. We can remove the awkward logic that affects the trigger behavior
   based on the gcMode. Now gcMode purely controls whether to run a
   STW GC or not and the gcTrigger controls whether this is a forced
   GC that cannot be consolidated with other GC cycles.

2. We can lift the time-based triggering logic in sysmon to just
   another type of GC trigger and move the logic to the trigger test.

3. This sets us up to have a cycle count-based trigger, which we'll
   use to make runtime.GC trigger concurrent GC with the desired
   consolidation properties.

For #18216.

Change-Id: If9cd49349579a548800f5022ae47b8128004bbfc
Reviewed-on: https://go-review.googlesource.com/37516
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index ae19120a31..dae8f135bc 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -228,7 +228,8 @@ func forcegchelper() {
 		if debug.gctrace > 0 {
 			println("GC forced")
 		}
-		gcStart(gcBackgroundMode, true)
+		// Time-triggered, fully concurrent.
+		gcStart(gcBackgroundMode, gcTrigger{kind: gcTriggerTime, now: nanotime()})
 	}
 }
 
@@ -3790,8 +3791,7 @@ func sysmon() {
 			idle++
 		}
 		// check if we need to force a GC
-		lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
-		if gcShouldStart(true) && lastgc != 0 && now-lastgc > forcegcperiod && atomic.Load(&forcegc.idle) != 0 {
+		if t := (gcTrigger{kind: gcTriggerTime, now: now}); t.test() && atomic.Load(&forcegc.idle) != 0 {
 			lock(&forcegc.lock)
 			forcegc.idle = 0
 			forcegc.g.schedlink = 0