runtime: refactor runtime->tracer API to appear more like a lock

Currently the execution tracer synchronizes with itself using very
heavyweight operations. As a result, it's totally fine for most of the
tracer code to look like:

    if traceEnabled() {
	traceXXX(...)
    }

However, if we want to make that synchronization more lightweight (as
issue #60773 proposes), then this is insufficient. In particular, we
need to make sure the tracer can't observe an inconsistency between g
atomicstatus and the event that would be emitted for a particular
g transition. This means making the g status change appear to happen
atomically with the corresponding trace event being written out from the
perspective of the tracer.

This requires a change in API to something more like a lock. While we're
here, we might as well make sure that trace events can *only* be emitted
while this lock is held. This change introduces such an API:
traceAcquire, which returns a value that can emit events, and
traceRelease, which requires the value that was returned by
traceAcquire. In practice, this won't be a real lock, it'll be more like
a seqlock.

For the current tracer, this API is completely overkill and the value
returned by traceAcquire basically just checks trace.enabled. But it's
necessary for the tracer described in #60773 and we can implement that
more cleanly if we do this refactoring now instead of later.

For #60773.

Change-Id: Ibb9ff5958376339fafc2b5180aef65cf2ba18646
Reviewed-on: https://go-review.googlesource.com/c/go/+/515635
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

Author: mknyszek

src/runtime/debugcall.go

@@ -166,10 +166,12 @@ func debugCallWrap(dispatch uintptr) {
 		gp.schedlink = 0
 
 		// Park the calling goroutine.
-		if traceEnabled() {
-			traceGoPark(traceBlockDebugCall, 1)
-		}
+		trace := traceAcquire()
 		casGToWaiting(gp, _Grunning, waitReasonDebugCall)
+		if trace.ok() {
+			trace.GoPark(traceBlockDebugCall, 1)
+			traceRelease(trace)
+		}
 		dropg()
 
 		// Directly execute the new goroutine. The debug
@@ -225,19 +227,23 @@ func debugCallWrap1() {
 		// Switch back to the calling goroutine. At some point
 		// the scheduler will schedule us again and we'll
 		// finish exiting.
-		if traceEnabled() {
-			traceGoSched()
-		}
+		trace := traceAcquire()
 		casgstatus(gp, _Grunning, _Grunnable)
+		if trace.ok() {
+			trace.GoSched()
+			traceRelease(trace)
+		}
 		dropg()
 		lock(&sched.lock)
 		globrunqput(gp)
 		unlock(&sched.lock)
 
-		if traceEnabled() {
-			traceGoUnpark(callingG, 0)
-		}
+		trace = traceAcquire()
 		casgstatus(callingG, _Gwaiting, _Grunnable)
+		if trace.ok() {
+			trace.GoUnpark(callingG, 0)
+			traceRelease(trace)
+		}
 		execute(callingG, true)
 	})
 }

src/runtime/mcentral.go

@@ -84,8 +84,10 @@ func (c *mcentral) cacheSpan() *mspan {
 	deductSweepCredit(spanBytes, 0)
 
 	traceDone := false
-	if traceEnabled() {
-		traceGCSweepStart()
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.GCSweepStart()
+		traceRelease(trace)
 	}
 
 	// If we sweep spanBudget spans without finding any free
@@ -157,9 +159,11 @@ func (c *mcentral) cacheSpan() *mspan {
 		}
 		sweep.active.end(sl)
 	}
-	if traceEnabled() {
-		traceGCSweepDone()
+	trace = traceAcquire()
+	if trace.ok() {
+		trace.GCSweepDone()
 		traceDone = true
+		traceRelease(trace)
 	}
 
 	// We failed to get a span from the mcentral so get one from mheap.
@@ -170,8 +174,12 @@ func (c *mcentral) cacheSpan() *mspan {
 
 	// At this point s is a span that should have free slots.
 havespan:
-	if traceEnabled() && !traceDone {
-		traceGCSweepDone()
+	if !traceDone {
+		trace := traceAcquire()
+		if trace.ok() {
+			trace.GCSweepDone()
+			traceRelease(trace)
+		}
 	}
 	n := int(s.nelems) - int(s.allocCount)
 	if n == 0 || s.freeindex == s.nelems || s.allocCount == s.nelems {

src/runtime/mgc.go

@@ -647,8 +647,10 @@ func gcStart(trigger gcTrigger) {
 	// Update it under gcsema to avoid gctrace getting wrong values.
 	work.userForced = trigger.kind == gcTriggerCycle
 
-	if traceEnabled() {
-		traceGCStart()
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.GCStart()
+		traceRelease(trace)
 	}
 
 	// Check that all Ps have finished deferred mcache flushes.
@@ -989,8 +991,10 @@ func gcMarkTermination() {
 	mp.traceback = 0
 	casgstatus(curgp, _Gwaiting, _Grunning)
 
-	if traceEnabled() {
-		traceGCDone()
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.GCDone()
+		traceRelease(trace)
 	}
 
 	// all done

src/runtime/mgcmark.go

@@ -420,7 +420,11 @@ retry:
 		// If the CPU limiter is enabled, intentionally don't
 		// assist to reduce the amount of CPU time spent in the GC.
 		if traced {
-			traceGCMarkAssistDone()
+			trace := traceAcquire()
+			if trace.ok() {
+				trace.GCMarkAssistDone()
+				traceRelease(trace)
+			}
 		}
 		return
 	}
@@ -461,15 +465,22 @@ retry:
 			// We were able to steal all of the credit we
 			// needed.
 			if traced {
-				traceGCMarkAssistDone()
+				trace := traceAcquire()
+				if trace.ok() {
+					trace.GCMarkAssistDone()
+					traceRelease(trace)
+				}
 			}
 			return
 		}
 	}
-
 	if traceEnabled() && !traced {
-		traced = true
-		traceGCMarkAssistStart()
+		trace := traceAcquire()
+		if trace.ok() {
+			traced = true
+			trace.GCMarkAssistStart()
+			traceRelease(trace)
+		}
 	}
 
 	// Perform assist work
@@ -515,7 +526,11 @@ retry:
 		// this G's assist debt, or the GC cycle is over.
 	}
 	if traced {
-		traceGCMarkAssistDone()
+		trace := traceAcquire()
+		if trace.ok() {
+			trace.GCMarkAssistDone()
+			traceRelease(trace)
+		}
 	}
 }
 

src/runtime/mgcpacer.go

@@ -807,9 +807,11 @@ func (c *gcControllerState) findRunnableGCWorker(pp *p, now int64) (*g, int64) {
 
 	// Run the background mark worker.
 	gp := node.gp.ptr()
+	trace := traceAcquire()
 	casgstatus(gp, _Gwaiting, _Grunnable)
-	if traceEnabled() {
-		traceGoUnpark(gp, 0)
+	if trace.ok() {
+		trace.GoUnpark(gp, 0)
+		traceRelease(trace)
 	}
 	return gp, now
 }
@@ -828,8 +830,10 @@ func (c *gcControllerState) resetLive(bytesMarked uint64) {
 	c.triggered = ^uint64(0) // Reset triggered.
 
 	// heapLive was updated, so emit a trace event.
-	if traceEnabled() {
-		traceHeapAlloc(bytesMarked)
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.HeapAlloc(bytesMarked)
+		traceRelease(trace)
 	}
 }
 
@@ -856,10 +860,12 @@ func (c *gcControllerState) markWorkerStop(mode gcMarkWorkerMode, duration int64
 
 func (c *gcControllerState) update(dHeapLive, dHeapScan int64) {
 	if dHeapLive != 0 {
+		trace := traceAcquire()
 		live := gcController.heapLive.Add(dHeapLive)
-		if traceEnabled() {
+		if trace.ok() {
 			// gcController.heapLive changed.
-			traceHeapAlloc(live)
+			trace.HeapAlloc(live)
+			traceRelease(trace)
 		}
 	}
 	if gcBlackenEnabled == 0 {
@@ -1428,8 +1434,10 @@ func gcControllerCommit() {
 
 	// TODO(mknyszek): This isn't really accurate any longer because the heap
 	// goal is computed dynamically. Still useful to snapshot, but not as useful.
-	if traceEnabled() {
-		traceHeapGoal()
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.HeapGoal()
+		traceRelease(trace)
 	}
 
 	trigger, heapGoal := gcController.trigger()

src/runtime/mgcsweep.go

@@ -516,8 +516,10 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
 		throw("mspan.sweep: bad span state")
 	}
 
-	if traceEnabled() {
-		traceGCSweepSpan(s.npages * _PageSize)
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.GCSweepSpan(s.npages * _PageSize)
+		traceRelease(trace)
 	}
 
 	mheap_.pagesSwept.Add(int64(s.npages))
@@ -889,8 +891,10 @@ func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) {
 		return
 	}
 
-	if traceEnabled() {
-		traceGCSweepStart()
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.GCSweepStart()
+		traceRelease(trace)
 	}
 
 	// Fix debt if necessary.
@@ -929,8 +933,10 @@ retry:
 		}
 	}
 
-	if traceEnabled() {
-		traceGCSweepDone()
+	trace = traceAcquire()
+	if trace.ok() {
+		trace.GCSweepDone()
+		traceRelease(trace)
 	}
 }
 

src/runtime/mheap.go

@@ -791,8 +791,10 @@ func (h *mheap) reclaim(npage uintptr) {
 	// traceGCSweepStart/Done pair on the P.
 	mp := acquirem()
 
-	if traceEnabled() {
-		traceGCSweepStart()
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.GCSweepStart()
+		traceRelease(trace)
 	}
 
 	arenas := h.sweepArenas
@@ -839,8 +841,10 @@ func (h *mheap) reclaim(npage uintptr) {
 		unlock(&h.lock)
 	}
 
-	if traceEnabled() {
-		traceGCSweepDone()
+	trace = traceAcquire()
+	if trace.ok() {
+		trace.GCSweepDone()
+		traceRelease(trace)
 	}
 	releasem(mp)
 }
@@ -911,10 +915,12 @@ func (h *mheap) reclaimChunk(arenas []arenaIdx, pageIdx, n uintptr) uintptr {
 		n -= uintptr(len(inUse) * 8)
 	}
 	sweep.active.end(sl)
-	if traceEnabled() {
+	trace := traceAcquire()
+	if trace.ok() {
 		unlock(&h.lock)
 		// Account for pages scanned but not reclaimed.
-		traceGCSweepSpan((n0 - nFreed) * pageSize)
+		trace.GCSweepSpan((n0 - nFreed) * pageSize)
+		traceRelease(trace)
 		lock(&h.lock)
 	}