runtime: eliminate unnecessary ragged barrier

The ragged barrier after entering the concurrent mark phase is
vestigial. This used to be the point where we enabled write barriers,
so it was necessary to synchronize all Ps to ensure write barriers
were enabled before any marking occurred. However, we've long since
switched to enabling write barriers during the concurrent scan phase,
so the start-the-world at the beginning of the concurrent scan phase
ensures that all Ps have enabled the write barrier.

Hence, we can eliminate the old "install write barrier" phase.

Fixes #11971.

Change-Id: I8cdcb84b5525cef19927d51ea11ba0a4db991ea8
Reviewed-on: https://go-review.googlesource.com/16044
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -943,7 +943,7 @@ func backgroundgc() {
 func gc(mode gcMode) {
 	// Timing/utilization tracking
 	var stwprocs, maxprocs int32
-	var tSweepTerm, tScan, tInstallWB, tMark, tMarkTerm int64
+	var tSweepTerm, tScan, tMark, tMarkTerm int64
 
 	// debug.gctrace variables
 	var heap0, heap1, heap2, heapGoal uint64
@@ -996,31 +996,30 @@ func gc(mode gcMode) {
 		heapGoal = gcController.heapGoal
 
 		systemstack(func() {
-			// Enter scan phase. This enables write
-			// barriers to track changes to stack frames
-			// above the stack barrier.
+			// Enter scan phase and enable write barriers.
 			//
-			// TODO: This has evolved to the point where
-			// we carefully ensure invariants we no longer
-			// depend on. Either:
+			// Because the world is stopped, all Ps will
+			// observe that write barriers are enabled by
+			// the time we start the world and begin
+			// scanning.
 			//
-			// 1) Enable full write barriers for the scan,
-			// but eliminate the ragged barrier below
-			// (since the start the world ensures all Ps
-			// have observed the write barrier enable) and
-			// consider draining during the scan.
+			// It's necessary to enable write barriers
+			// during the scan phase for several reasons:
 			//
-			// 2) Only enable write barriers for writes to
-			// the stack at this point, and then enable
-			// write barriers for heap writes when we
-			// enter the mark phase. This means we cannot
-			// drain in the scan phase and must perform a
-			// ragged barrier to ensure all Ps have
-			// enabled heap write barriers before we drain
-			// or enable assists.
+			// They must be enabled for writes to higher
+			// stack frames before we scan stacks and
+			// install stack barriers because this is how
+			// we track writes to inactive stack frames.
+			// (Alternatively, we could not install stack
+			// barriers over frame boundaries with
+			// up-pointers).
 			//
-			// 3) Don't install stack barriers over frame
-			// boundaries where there are up-pointers.
+			// They must be enabled before assists are
+			// enabled because they must be enabled before
+			// any non-leaf heap objects are marked. Since
+			// allocations are blocked until assists can
+			// happen, we want enable assists as early as
+			// possible.
 			setGCPhase(_GCscan)
 
 			// markrootSpans uses work.spans, so make sure
@@ -1045,12 +1044,7 @@ func gc(mode gcMode) {
 			gcscan_m()
 
 			// Enter mark phase.
-			tInstallWB = nanotime()
 			setGCPhase(_GCmark)
-			// Ensure all Ps have observed the phase
-			// change and have write barriers enabled
-			// before any blackening occurs.
-			forEachP(func(*p) {})
 		})
 		// Concurrent mark.
 		tMark = nanotime()
@@ -1106,7 +1100,7 @@ func gc(mode gcMode) {
 		gcController.endCycle()
 	} else {
 		t := nanotime()
-		tScan, tInstallWB, tMark, tMarkTerm = t, t, t, t
+		tScan, tMark, tMarkTerm = t, t, t
 		heapGoal = heap0
 	}
 
@@ -1201,13 +1195,12 @@ func gc(mode gcMode) {
 
 	// Update work.totaltime.
 	sweepTermCpu := int64(stwprocs) * (tScan - tSweepTerm)
-	scanCpu := tInstallWB - tScan
-	installWBCpu := int64(0)
+	scanCpu := tMark - tScan
 	// We report idle marking time below, but omit it from the
 	// overall utilization here since it's "free".
 	markCpu := gcController.assistTime + gcController.dedicatedMarkTime + gcController.fractionalMarkTime
 	markTermCpu := int64(stwprocs) * (now - tMarkTerm)
-	cycleCpu := sweepTermCpu + scanCpu + installWBCpu + markCpu + markTermCpu
+	cycleCpu := sweepTermCpu + scanCpu + markCpu + markTermCpu
 	work.totaltime += cycleCpu
 
 	// Compute overall GC CPU utilization.
@@ -1226,6 +1219,10 @@ func gc(mode gcMode) {
 		tEnd := now
 		util := int(memstats.gc_cpu_fraction * 100)
 
+		// Install WB phase is no longer used.
+		tInstallWB := tMark
+		installWBCpu := int64(0)
+
 		var sbuf [24]byte
 		printlock()
 		print("gc ", memstats.numgc,