runtime: detangle sweeper pacing from GC pacing

The sweeper's pacing state is global, so detangle it from the GC pacer's
state updates so that the GC pacer can be tested.

For #44167.

Change-Id: Ibcea989cd435b73c5891f777d9f95f9604e03bd1
Reviewed-on: https://go-review.googlesource.com/c/go/+/309273
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/mgc.go

@@ -971,6 +971,7 @@ func gcMarkTermination(nextTriggerRatio float64) {
 
 	// Update GC trigger and pacing for the next cycle.
 	gcController.commit(nextTriggerRatio)
+	gcPaceSweeper(gcController.trigger)
 	gcPaceScavenger(gcController.heapGoal, gcController.lastHeapGoal)
 
 	// Update timing memstats

src/runtime/mgcpacer.go

@@ -735,40 +735,6 @@ func (c *gcControllerState) commit(triggerRatio float64) {
 	if gcphase != _GCoff {
 		c.revise()
 	}
-
-	// Update sweep pacing.
-	if isSweepDone() {
-		mheap_.sweepPagesPerByte = 0
-	} else {
-		// Concurrent sweep needs to sweep all of the in-use
-		// pages by the time the allocated heap reaches the GC
-		// trigger. Compute the ratio of in-use pages to sweep
-		// per byte allocated, accounting for the fact that
-		// some might already be swept.
-		heapLiveBasis := atomic.Load64(&c.heapLive)
-		heapDistance := int64(trigger) - int64(heapLiveBasis)
-		// Add a little margin so rounding errors and
-		// concurrent sweep are less likely to leave pages
-		// unswept when GC starts.
-		heapDistance -= 1024 * 1024
-		if heapDistance < _PageSize {
-			// Avoid setting the sweep ratio extremely high
-			heapDistance = _PageSize
-		}
-		pagesSwept := mheap_.pagesSwept.Load()
-		pagesInUse := mheap_.pagesInUse.Load()
-		sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
-		if sweepDistancePages <= 0 {
-			mheap_.sweepPagesPerByte = 0
-		} else {
-			mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
-			mheap_.sweepHeapLiveBasis = heapLiveBasis
-			// Write pagesSweptBasis last, since this
-			// signals concurrent sweeps to recompute
-			// their debt.
-			mheap_.pagesSweptBasis.Store(pagesSwept)
-		}
-	}
 }
 
 // effectiveGrowthRatio returns the current effective heap growth
@@ -819,6 +785,7 @@ func setGCPercent(in int32) (out int32) {
 	systemstack(func() {
 		lock(&mheap_.lock)
 		out = gcController.setGCPercent(in)
+		gcPaceSweeper(gcController.trigger)
 		gcPaceScavenger(gcController.heapGoal, gcController.lastHeapGoal)
 		unlock(&mheap_.lock)
 	})

src/runtime/mgcsweep.go

@@ -830,3 +830,46 @@ func clobberfree(x unsafe.Pointer, size uintptr) {
 		*(*uint32)(add(x, i)) = 0xdeadbeef
 	}
 }
+
+// gcPaceSweeper updates the sweeper's pacing parameters.
+//
+// Must be called whenever the GC's pacing is updated.
+//
+// The world must be stopped, or mheap_.lock must be held.
+func gcPaceSweeper(trigger uint64) {
+	assertWorldStoppedOrLockHeld(&mheap_.lock)
+
+	// Update sweep pacing.
+	if isSweepDone() {
+		mheap_.sweepPagesPerByte = 0
+	} else {
+		// Concurrent sweep needs to sweep all of the in-use
+		// pages by the time the allocated heap reaches the GC
+		// trigger. Compute the ratio of in-use pages to sweep
+		// per byte allocated, accounting for the fact that
+		// some might already be swept.
+		heapLiveBasis := atomic.Load64(&gcController.heapLive)
+		heapDistance := int64(trigger) - int64(heapLiveBasis)
+		// Add a little margin so rounding errors and
+		// concurrent sweep are less likely to leave pages
+		// unswept when GC starts.
+		heapDistance -= 1024 * 1024
+		if heapDistance < _PageSize {
+			// Avoid setting the sweep ratio extremely high
+			heapDistance = _PageSize
+		}
+		pagesSwept := mheap_.pagesSwept.Load()
+		pagesInUse := mheap_.pagesInUse.Load()
+		sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
+		if sweepDistancePages <= 0 {
+			mheap_.sweepPagesPerByte = 0
+		} else {
+			mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
+			mheap_.sweepHeapLiveBasis = heapLiveBasis
+			// Write pagesSweptBasis last, since this
+			// signals concurrent sweeps to recompute
+			// their debt.
+			mheap_.pagesSweptBasis.Store(pagesSwept)
+		}
+	}
+}