runtime: add a more stable isSystemGoroutine mode

Currently, isSystemGoroutine varies on whether it considers the
finalizer goroutine a user goroutine or a system goroutine. For the
next CL, we're going to want to always consider the finalier goroutine
a user goroutine, so add a flag that indicates that.

Updates #26903. This is preparation for unifying STW GC and concurrent
GC.

Change-Id: Iafc92e519c13d9f8d879332cb5f0d12164104c33
Reviewed-on: https://go-review.googlesource.com/c/134778
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/heapdump.go

@@ -346,7 +346,7 @@ func dumpgoroutine(gp *g) {
 	dumpint(uint64(gp.goid))
 	dumpint(uint64(gp.gopc))
 	dumpint(uint64(readgstatus(gp)))
-	dumpbool(isSystemGoroutine(gp))
+	dumpbool(isSystemGoroutine(gp, false))
 	dumpbool(false) // isbackground
 	dumpint(uint64(gp.waitsince))
 	dumpstr(gp.waitreason.String())

src/runtime/mprof.go

@@ -723,7 +723,7 @@ func GoroutineProfile(p []StackRecord) (n int, ok bool) {
 	isOK := func(gp1 *g) bool {
 		// Checking isSystemGoroutine here makes GoroutineProfile
 		// consistent with both NumGoroutine and Stack.
-		return gp1 != gp && readgstatus(gp1) != _Gdead && !isSystemGoroutine(gp1)
+		return gp1 != gp && readgstatus(gp1) != _Gdead && !isSystemGoroutine(gp1, false)
 	}
 
 	stopTheWorld("profile")

src/runtime/proc.go

@@ -2730,7 +2730,7 @@ func goexit0(gp *g) {
 	_g_ := getg()
 
 	casgstatus(gp, _Grunning, _Gdead)
-	if isSystemGoroutine(gp) {
+	if isSystemGoroutine(gp, false) {
 		atomic.Xadd(&sched.ngsys, -1)
 	}
 	gp.m = nil
@@ -3381,7 +3381,7 @@ func newproc1(fn *funcval, argp *uint8, narg int32, callergp *g, callerpc uintpt
 	if _g_.m.curg != nil {
 		newg.labels = _g_.m.curg.labels
 	}
-	if isSystemGoroutine(newg) {
+	if isSystemGoroutine(newg, false) {
 		atomic.Xadd(&sched.ngsys, +1)
 	}
 	newg.gcscanvalid = false
@@ -4244,7 +4244,7 @@ func checkdead() {
 	lock(&allglock)
 	for i := 0; i < len(allgs); i++ {
 		gp := allgs[i]
-		if isSystemGoroutine(gp) {
+		if isSystemGoroutine(gp, false) {
 			continue
 		}
 		s := readgstatus(gp)

src/runtime/traceback.go

@@ -945,7 +945,7 @@ func tracebackothers(me *g) {
 
 	lock(&allglock)
 	for _, gp := range allgs {
-		if gp == me || gp == g.m.curg || readgstatus(gp) == _Gdead || isSystemGoroutine(gp) && level < 2 {
+		if gp == me || gp == g.m.curg || readgstatus(gp) == _Gdead || isSystemGoroutine(gp, false) && level < 2 {
 			continue
 		}
 		print("\n")
@@ -1031,7 +1031,11 @@ func topofstack(f funcInfo, g0 bool) bool {
 // in stack dumps and deadlock detector. This is any goroutine that
 // starts at a runtime.* entry point, except for runtime.main and
 // sometimes runtime.runfinq.
-func isSystemGoroutine(gp *g) bool {
+//
+// If fixed is true, any goroutine that can vary between user and
+// system (that is, the finalizer goroutine) is considered a user
+// goroutine.
+func isSystemGoroutine(gp *g, fixed bool) bool {
 	// Keep this in sync with cmd/trace/trace.go:isSystemGoroutine.
 	f := findfunc(gp.startpc)
 	if !f.valid() {
@@ -1043,6 +1047,11 @@ func isSystemGoroutine(gp *g) bool {
 	if f.funcID == funcID_runfinq {
 		// We include the finalizer goroutine if it's calling
 		// back into user code.
+		if fixed {
+			// This goroutine can vary. In fixed mode,
+			// always consider it a user goroutine.
+			return false
+		}
 		return !fingRunning
 	}
 	return hasPrefix(funcname(f), "runtime.")