runtime: report finalizer and cleanup queue length with checkfinalizer>0

This change adds tracking for approximate finalizer and cleanup queue
lengths. These lengths are reported once every GC cycle as a single line
printed to stderr when GODEBUG=checkfinalizer>0.

This change lays the groundwork for runtime/metrics metrics to produce
the same values.

For #72948.
For #72950.

Change-Id: I081721238a0fc4c7e5bee2dbaba6cfb4120d1a33
Reviewed-on: https://go-review.googlesource.com/c/go/+/671437
Reviewed-by: Michael Pratt <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: mknyszek

src/runtime/mcleanup.go

@@ -336,6 +336,20 @@ type cleanupQueue struct {
 	//
 	// Read without lock, written only with lock held.
 	needg atomic.Uint32
+
+	// Cleanup queue stats.
+
+	// queued represents a monotonic count of queued cleanups. This is sharded across
+	// Ps via the field cleanupsQueued in each p, so reading just this value is insufficient.
+	// In practice, this value only includes the queued count of dead Ps.
+	//
+	// Writes are protected by STW.
+	queued uint64
+
+	// executed is a monotonic count of executed cleanups.
+	//
+	// Read and updated atomically.
+	executed atomic.Uint64
 }
 
 // addWork indicates that n units of parallelizable work have been added to the queue.
@@ -387,6 +401,7 @@ func (q *cleanupQueue) enqueue(fn *funcval) {
 		pp.cleanups = nil
 		q.addWork(1)
 	}
+	pp.cleanupsQueued++
 	releasem(mp)
 }
 
@@ -586,6 +601,19 @@ func (q *cleanupQueue) endRunningCleanups() {
 	releasem(mp)
 }
 
+func (q *cleanupQueue) readQueueStats() (queued, executed uint64) {
+	executed = q.executed.Load()
+	queued = q.queued
+
+	// N.B. This is inconsistent, but that's intentional. It's just an estimate.
+	// Read this _after_ reading executed to decrease the chance that we observe
+	// an inconsistency in the statistics (executed > queued).
+	for _, pp := range allp {
+		queued += pp.cleanupsQueued
+	}
+	return
+}
+
 func maxCleanupGs() uint32 {
 	// N.B. Left as a function to make changing the policy easier.
 	return uint32(max(gomaxprocs/4, 1))
@@ -636,6 +664,7 @@ func runCleanups() {
 			}
 		}
 		gcCleanups.endRunningCleanups()
+		gcCleanups.executed.Add(int64(b.n))
 
 		atomic.Store(&b.n, 0) // Synchronize with markroot. See comment in cleanupBlockHeader.
 		gcCleanups.free.push(&b.lfnode)

src/runtime/mfinal.go

@@ -44,11 +44,13 @@ const (
 )
 
 var (
-	finlock    mutex     // protects the following variables
-	fing       *g        // goroutine that runs finalizers
-	finq       *finBlock // list of finalizers that are to be executed
-	finc       *finBlock // cache of free blocks
-	finptrmask [finBlockSize / goarch.PtrSize / 8]byte
+	finlock     mutex     // protects the following variables
+	fing        *g        // goroutine that runs finalizers
+	finq        *finBlock // list of finalizers that are to be executed
+	finc        *finBlock // cache of free blocks
+	finptrmask  [finBlockSize / goarch.PtrSize / 8]byte
+	finqueued   uint64 // monotonic count of queued finalizers
+	finexecuted uint64 // monotonic count of executed finalizers
 )
 
 var allfin *finBlock // list of all blocks
@@ -108,6 +110,7 @@ func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot
 	}
 
 	lock(&finlock)
+
 	if finq == nil || finq.cnt == uint32(len(finq.fin)) {
 		if finc == nil {
 			finc = (*finBlock)(persistentalloc(finBlockSize, 0, &memstats.gcMiscSys))
@@ -141,6 +144,7 @@ func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot
 	f.fint = fint
 	f.ot = ot
 	f.arg = p
+	finqueued++
 	unlock(&finlock)
 	fingStatus.Or(fingWake)
 }
@@ -177,6 +181,14 @@ func finalizercommit(gp *g, lock unsafe.Pointer) bool {
 	return true
 }
 
+func finReadQueueStats() (queued, executed uint64) {
+	lock(&finlock)
+	queued = finqueued
+	executed = finexecuted
+	unlock(&finlock)
+	return
+}
+
 // This is the goroutine that runs all of the finalizers.
 func runFinalizers() {
 	var (
@@ -204,7 +216,8 @@ func runFinalizers() {
 			racefingo()
 		}
 		for fb != nil {
-			for i := fb.cnt; i > 0; i-- {
+			n := fb.cnt
+			for i := n; i > 0; i-- {
 				f := &fb.fin[i-1]
 
 				var regs abi.RegArgs
@@ -270,6 +283,7 @@ func runFinalizers() {
 			}
 			next := fb.next
 			lock(&finlock)
+			finexecuted += uint64(n)
 			fb.next = finc
 			finc = fb
 			unlock(&finlock)

src/runtime/mgc.go

@@ -1337,6 +1337,19 @@ func gcMarkTermination(stw worldStop) {
 		printunlock()
 	}
 
+	// Print finalizer/cleanup queue length. Like gctrace, do this before the next GC starts.
+	// The fact that the next GC might start is not that problematic here, but acts as a convenient
+	// lock on printing this information (so it cannot overlap with itself from the next GC cycle).
+	if debug.checkfinalizers > 0 {
+		fq, fe := finReadQueueStats()
+		fn := max(int64(fq)-int64(fe), 0)
+
+		cq, ce := gcCleanups.readQueueStats()
+		cn := max(int64(cq)-int64(ce), 0)
+
+		println("checkfinalizers: queue:", fn, "finalizers +", cn, "cleanups")
+	}
+
 	// Set any arena chunks that were deferred to fault.
 	lock(&userArenaState.lock)
 	faultList := userArenaState.fault

src/runtime/proc.go

@@ -5743,6 +5743,8 @@ func (pp *p) destroy() {
 		pp.raceprocctx = 0
 	}
 	pp.gcAssistTime = 0
+	gcCleanups.queued += pp.cleanupsQueued
+	pp.cleanupsQueued = 0
 	pp.status = _Pdead
 }
 

src/runtime/runtime2.go

@@ -732,7 +732,8 @@ type p struct {
 	timers timers
 
 	// Cleanups.
-	cleanups *cleanupBlock
+	cleanups       *cleanupBlock
+	cleanupsQueued uint64 // monotonic count of cleanups queued by this P
 
 	// maxStackScanDelta accumulates the amount of stack space held by
 	// live goroutines (i.e. those eligible for stack scanning).