runtime: assist before allocating

Currently, when the mutator allocates, the runtime first allocates the
memory and then, if that G has done "enough" allocation, the runtime
checks whether the G has assist debt to pay off and, if so, pays it
off. This approach leads to under-assisting, where a G can allocate a
large region (or many small regions) before paying for it, or can even
exit with outstanding debt.

This commit flips this around so that a G always acquires enough
credit for an allocation before it can perform that allocation. We
continue to amortize the cost of assists by requiring that they
over-assist when triggered to build up credit for many allocations.

Fixes #11967.

Change-Id: Idac9f11133b328535667674d837be72c23ebd899
Reviewed-on: https://go-review.googlesource.com/15409Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>

src/runtime/malloc.go

@@ -510,6 +510,27 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 		return persistentalloc(size, align, &memstats.other_sys)
 	}
 
+	// assistG is the G to charge for this allocation, or nil if
+	// GC is not currently active.
+	var assistG *g
+	if gcBlackenEnabled != 0 {
+		// Charge the current user G for this allocation.
+		assistG = getg()
+		if assistG.m.curg != nil {
+			assistG = assistG.m.curg
+		}
+		// Charge the allocation against the G. We'll account
+		// for internal fragmentation at the end of mallocgc.
+		assistG.gcAssistBytes -= int64(size)
+
+		if assistG.gcAssistBytes < 0 {
+			// This G is in debt. Assist the GC to correct
+			// this before allocating. This must happen
+			// before disabling preemption.
+			gcAssistAlloc(assistG)
+		}
+	}
+
 	// Set mp.mallocing to keep from being preempted by GC.
 	mp := acquirem()
 	if mp.mallocing != 0 {
@@ -704,15 +725,15 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 		}
 	}
 
+	if assistG != nil {
+		// Account for internal fragmentation in the assist
+		// debt now that we know it.
+		assistG.gcAssistBytes -= int64(size - dataSize)
+	}
+
 	if shouldhelpgc && shouldtriggergc() {
 		startGC(gcBackgroundMode, false)
-	} else if gcBlackenEnabled != 0 {
-		// Assist garbage collector. We delay this until the
-		// epilogue so that it doesn't interfere with the
-		// inner working of malloc such as mcache refills that
-		// might happen while doing the gcAssistAlloc.
-		gcAssistAlloc(size, shouldhelpgc)
-	} else if shouldhelpgc && bggc.working != 0 {
+	} else if shouldhelpgc && bggc.working != 0 && gcBlackenEnabled == 0 {
 		// The GC is starting up or shutting down, so we can't
 		// assist, but we also can't allocate unabated. Slow
 		// down this G's allocation and help the GC stay

src/runtime/mgc.go

@@ -723,6 +723,12 @@ const gcCreditSlack = 2000
 // can accumulate on a P before updating gcController.assistTime.
 const gcAssistTimeSlack = 5000
 
+// gcOverAssistBytes determines how many extra allocation bytes of
+// assist credit a GC assist builds up when an assist happens. This
+// amortizes the cost of an assist by pre-paying for this many bytes
+// of future allocations.
+const gcOverAssistBytes = 1 << 20
+
 // Determine whether to initiate a GC.
 // If the GC is already working no need to trigger another one.
 // This should establish a feedback loop where if the GC does not

src/runtime/mgcmark.go

@@ -198,28 +198,12 @@ func markrootSpans(gcw *gcWork, shard int) {
 	}
 }
 
-// gcAssistAlloc records and allocation of size bytes and, if
-// allowAssist is true, may assist GC scanning in proportion to the
-// allocations performed by this mutator since the last assist.
+// gcAssistAlloc performs GC work to make gp's assist debt positive.
+// gp must be the calling user gorountine.
 //
-// It should only be called if gcBlackenEnabled != 0.
-//
-// This must be called with preemption disabled.
+// This must be called with preemption enabled.
 //go:nowritebarrier
-func gcAssistAlloc(size uintptr, allowAssist bool) {
-	// Find the G responsible for this assist.
-	gp := getg()
-	if gp.m.curg != nil {
-		gp = gp.m.curg
-	}
-
-	// Record allocation.
-	gp.gcAssistBytes -= int64(size)
-
-	if !allowAssist || gp.gcAssistBytes >= 0 {
-		return
-	}
-
+func gcAssistAlloc(gp *g) {
 	// Don't assist in non-preemptible contexts. These are
 	// generally fragile and won't allow the assist to block.
 	if getg() == gp.m.g0 {
@@ -230,8 +214,9 @@ func gcAssistAlloc(size uintptr, allowAssist bool) {
 	}
 
 	// Compute the amount of scan work we need to do to make the
-	// balance positive.
-	debtBytes := -gp.gcAssistBytes
+	// balance positive. We over-assist to build up credit for
+	// future allocations and amortize the cost of assisting.
+	debtBytes := -gp.gcAssistBytes + gcOverAssistBytes
 	scanWork := int64(gcController.assistWorkPerByte * float64(debtBytes))
 
 retry:
@@ -358,7 +343,12 @@ retry:
 		// more, so go around again after performing an
 		// interruptible sleep for 100 us (the same as the
 		// getfull barrier) to let other mutators run.
+
+		// timeSleep may allocate, so avoid recursive assist.
+		gcAssistBytes := gp.gcAssistBytes
+		gp.gcAssistBytes = int64(^uint64(0) >> 1)
 		timeSleep(100 * 1000)
+		gp.gcAssistBytes = gcAssistBytes
 		goto retry
 	}
 }