runtime: reduce one STW when obtaining goroutine configuration file

Fixes #54014

Change-Id: If4ee2752008729e1ed4b767cfda52effdcec4959
GitHub-Last-Rev: 5ce300bf51
GitHub-Pull-Request: golang/go#58239
Reviewed-on: https://go-review.googlesource.com/c/go/+/464349
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: qiulaidongfeng <2645477756@qq.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mprof.go

@@ -1131,6 +1131,11 @@ func (p *goroutineProfileStateHolder) CompareAndSwap(old, new goroutineProfileSt
 	return (*atomic.Uint32)(p).CompareAndSwap(uint32(old), uint32(new))
 }
 
+//go:linkname runtime_gcount runtime/pprof.runtime_gcount
+func runtime_gcount() int {
+	return int(gcount())
+}
+
 func goroutineProfileWithLabelsConcurrent(p []StackRecord, labels []unsafe.Pointer) (n int, ok bool) {
 	semacquire(&goroutineProfile.sema)
 

src/runtime/pprof/pprof.go

@@ -755,6 +755,9 @@ func writeGoroutineStacks(w io.Writer) error {
 	return err
 }
 
+// runtime_gcount is defined in runtime/mprof.go
+func runtime_gcount() (n int)
+
 func writeRuntimeProfile(w io.Writer, debug int, name string, fetch func([]runtime.StackRecord, []unsafe.Pointer) (int, bool)) error {
 	// Find out how many records there are (fetch(nil)),
 	// allocate that many records, and get the data.
@@ -764,7 +767,13 @@ func writeRuntimeProfile(w io.Writer, debug int, name string, fetch func([]runti
 	// The loop should only execute one iteration in the common case.
 	var p []runtime.StackRecord
 	var labels []unsafe.Pointer
-	n, ok := fetch(nil, nil)
+	var n, ok = 0, false
+	if name == "goroutine" {
+		n = runtime_gcount()
+	} else {
+		n, ok = fetch(nil, nil)
+	}
+
 	for {
 		// Allocate room for a slightly bigger profile,
 		// in case a few more entries have been added