runtime: make traceStack testable and add a benchmark

Change-Id: Ide4daa5eee3fd4f3007d6ef23aa84b8916562c39
Reviewed-on: https://go-review.googlesource.com/c/go/+/684457
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: mknyszek

src/runtime/export_test.go

@@ -1917,3 +1917,13 @@ const (
 	BubbleAssocCurrentBubble = bubbleAssocCurrentBubble
 	BubbleAssocOtherBubble   = bubbleAssocOtherBubble
 )
+
+type TraceStackTable traceStackTable
+
+func (t *TraceStackTable) Reset() {
+	t.tab.reset()
+}
+
+func TraceStack(gp *G, tab *TraceStackTable) {
+	traceStack(0, gp, (*traceStackTable)(tab))
+}

src/runtime/symtab.go

@@ -981,6 +981,9 @@ func pcvalue(f funcInfo, off uint32, targetpc uintptr, strict bool) (int32, uint
 	// matches the cached contents.
 	const debugCheckCache = false
 
+	// If true, skip checking the cache entirely.
+	const skipCache = false
+
 	if off == 0 {
 		return -1, 0
 	}
@@ -991,7 +994,7 @@ func pcvalue(f funcInfo, off uint32, targetpc uintptr, strict bool) (int32, uint
 	var checkVal int32
 	var checkPC uintptr
 	ck := pcvalueCacheKey(targetpc)
-	{
+	if !skipCache {
 		mp := acquirem()
 		cache := &mp.pcvalueCache
 		// The cache can be used by the signal handler on this M. Avoid

src/runtime/trace.go

@@ -396,7 +396,7 @@ func traceAdvance(stopTrace bool) {
 				ug.status = readgstatus(s.g) &^ _Gscan
 				ug.waitreason = s.g.waitreason
 				ug.inMarkAssist = s.g.inMarkAssist
-				ug.stackID = traceStack(0, gp, gen)
+				ug.stackID = traceStack(0, gp, &trace.stackTab[gen%2])
 			}
 			resumeG(s)
 			casgstatus(me, _Gwaiting, _Grunning)

src/runtime/traceevent.go

@@ -56,7 +56,7 @@ func (e traceEventWriter) event(ev tracev2.EventType, args ...traceArg) {
 // It then returns a traceArg representing that stack which may be
 // passed to write.
 func (tl traceLocker) stack(skip int) traceArg {
-	return traceArg(traceStack(skip, nil, tl.gen))
+	return traceArg(traceStack(skip, nil, &trace.stackTab[tl.gen%2]))
 }
 
 // startPC takes a start PC for a goroutine and produces a unique

src/runtime/tracestack.go

@@ -28,10 +28,8 @@ const (
 // skip controls the number of leaf frames to omit in order to hide tracer internals
 // from stack traces, see CL 5523.
 //
-// Avoid calling this function directly. gen needs to be the current generation
-// that this stack trace is being written out for, which needs to be synchronized with
-// generations moving forward. Prefer traceEventWriter.stack.
-func traceStack(skip int, gp *g, gen uintptr) uint64 {
+// Avoid calling this function directly. Prefer traceEventWriter.stack.
+func traceStack(skip int, gp *g, tab *traceStackTable) uint64 {
 	var pcBuf [tracev2.MaxFramesPerStack]uintptr
 
 	// Figure out gp and mp for the backtrace.
@@ -134,7 +132,7 @@ func traceStack(skip int, gp *g, gen uintptr) uint64 {
 	if nstk > 0 && gp.goid == 1 {
 		nstk-- // skip runtime.main
 	}
-	id := trace.stackTab[gen%2].put(pcBuf[:nstk])
+	id := tab.put(pcBuf[:nstk])
 	return id
 }
 

src/runtime/tracestack_test.go

@@ -0,0 +1,46 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"runtime"
+	"strconv"
+	"testing"
+)
+
+func BenchmarkTraceStack(b *testing.B) {
+	for _, stackDepth := range []int{1, 10, 100} {
+		b.Run("stackDepth="+strconv.Itoa(stackDepth), func(b *testing.B) {
+			benchmarkTraceStack(b, stackDepth)
+		})
+	}
+}
+
+func benchmarkTraceStack(b *testing.B, stackDepth int) {
+	var tab runtime.TraceStackTable
+	defer tab.Reset()
+
+	wait := make(chan struct{})
+	ready := make(chan struct{})
+	done := make(chan struct{})
+	var gp *runtime.G
+	go func() {
+		gp = runtime.Getg()
+		useStackAndCall(stackDepth, func() {
+			ready <- struct{}{}
+			<-wait
+		})
+		done <- struct{}{}
+	}()
+	<-ready
+
+	for b.Loop() {
+		runtime.TraceStack(gp, &tab)
+	}
+
+	// Clean up.
+	wait <- struct{}{}
+	<-done
+}