runtime: remove work.pauseStart

Most of the uses of work.pauseStart are completely useless, it could
simply be a local variable. One use passes a parameter from gcMarkDone
to gcMarkTermination, but that could simply be an argument.

Keeping this field in workType makes it seems more important than it
really is, so just drop it.

Change-Id: I2fdc0b21f8844e5e7be47148c3e10f13e49815c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/542075
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/mgc.go

@@ -419,7 +419,6 @@ type workType struct {
 	tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start
 
 	pauseNS    int64 // total STW time this cycle
-	pauseStart int64 // nanotime() of last STW
 
 	// debug.gctrace heap sizes for this cycle.
 	heap0, heap1, heap2 uint64
@@ -677,7 +676,7 @@ func gcStart(trigger gcTrigger) {
 
 	now := nanotime()
 	work.tSweepTerm = now
-	work.pauseStart = now
+	pauseStart := now
 	systemstack(func() { stopTheWorldWithSema(stwGCSweepTerm) })
 	// Finish sweep before we start concurrent scan.
 	systemstack(func() {
@@ -744,9 +743,9 @@ func gcStart(trigger gcTrigger) {
 	// Concurrent mark.
 	systemstack(func() {
 		now = startTheWorldWithSema()
-		work.pauseNS += now - work.pauseStart
+		work.pauseNS += now - pauseStart
 		work.tMark = now
-		memstats.gcPauseDist.record(now - work.pauseStart)
+		memstats.gcPauseDist.record(now - pauseStart)
 
 		sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm)
 		work.cpuStats.gcPauseTime += sweepTermCpu
@@ -858,7 +857,7 @@ top:
 	// shaded. Transition to mark termination.
 	now := nanotime()
 	work.tMarkTerm = now
-	work.pauseStart = now
+	pauseStart := now
 	getg().m.preemptoff = "gcing"
 	systemstack(func() { stopTheWorldWithSema(stwGCMarkTerm) })
 	// The gcphase is _GCmark, it will transition to _GCmarktermination
@@ -888,8 +887,8 @@ top:
 		getg().m.preemptoff = ""
 		systemstack(func() {
 			now := startTheWorldWithSema()
-			work.pauseNS += now - work.pauseStart
-			memstats.gcPauseDist.record(now - work.pauseStart)
+			work.pauseNS += now - pauseStart
+			memstats.gcPauseDist.record(now - pauseStart)
 		})
 		semrelease(&worldsema)
 		goto top
@@ -923,12 +922,12 @@ top:
 	gcController.endCycle(now, int(gomaxprocs), work.userForced)
 
 	// Perform mark termination. This will restart the world.
-	gcMarkTermination()
+	gcMarkTermination(pauseStart)
 }
 
 // World must be stopped and mark assists and background workers must be
 // disabled.
-func gcMarkTermination() {
+func gcMarkTermination(pauseStart int64) {
 	// Start marktermination (write barrier remains enabled for now).
 	setGCPhase(_GCmarktermination)
 
@@ -1009,9 +1008,9 @@ func gcMarkTermination() {
 	now := nanotime()
 	sec, nsec, _ := time_now()
 	unixNow := sec*1e9 + int64(nsec)
-	work.pauseNS += now - work.pauseStart
+	work.pauseNS += now - pauseStart
 	work.tEnd = now
-	memstats.gcPauseDist.record(now - work.pauseStart)
+	memstats.gcPauseDist.record(now - pauseStart)
 	atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user
 	atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us
 	memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS)