runtime: set G wait reason more consistently

Currently, wait reasons are set somewhat inconsistently. In a follow-up
CL, we're going to want to rely on the wait reason being there for
casgstatus, so the status quo isn't really going to work for that. Plus
this inconsistency means there are a whole bunch of cases where we could
be more specific about the G's status but aren't.

So, this change adds a new function, casGToWaiting which is like
casgstatus but also sets the wait reason. The goal is that by using this
API it'll be harder to forget to set a wait reason (or the lack thereof
will at least be explicit). This change then updates all casgstatus(gp,
..., _Gwaiting) calls to casGToWaiting(gp, ..., waitReasonX) instead.
For a number of these cases, we're missing a wait reason, and it
wouldn't hurt to add a wait reason for them, so this change also adds
those wait reasons.

For #49881.

Change-Id: Ia95e06ecb74ed17bb7bb94f1a362ebfe6bec1518
Reviewed-on: https://go-review.googlesource.com/c/go/+/427617
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/debugcall.go

@@ -158,11 +158,10 @@ func debugCallWrap(dispatch uintptr) {
 		gp.schedlink = 0
 
 		// Park the calling goroutine.
-		gp.waitreason = waitReasonDebugCall
 		if trace.enabled {
 			traceGoPark(traceEvGoBlock, 1)
 		}
-		casgstatus(gp, _Grunning, _Gwaiting)
+		casGToWaiting(gp, _Grunning, waitReasonDebugCall)
 		dropg()
 
 		// Directly execute the new goroutine. The debug

src/runtime/heapdump.go

@@ -691,8 +691,7 @@ func writeheapdump_m(fd uintptr, m *MemStats) {
 	assertWorldStopped()
 
 	gp := getg()
-	casgstatus(gp.m.curg, _Grunning, _Gwaiting)
-	gp.waitreason = waitReasonDumpingHeap
+	casGToWaiting(gp.m.curg, _Grunning, waitReasonDumpingHeap)
 
 	// Set dump file.
 	dumpfd = fd

src/runtime/mgc.go

@@ -810,7 +810,7 @@ top:
 		// Otherwise, our attempt to force all P's to a safepoint could
 		// result in a deadlock as we attempt to preempt a worker that's
 		// trying to preempt us (e.g. for a stack scan).
-		casgstatus(gp, _Grunning, _Gwaiting)
+		casGToWaiting(gp, _Grunning, waitReasonGCMarkTermination)
 		forEachP(func(pp *p) {
 			// Flush the write barrier buffer, since this may add
 			// work to the gcWork.
@@ -931,8 +931,7 @@ func gcMarkTermination() {
 	mp.preemptoff = "gcing"
 	mp.traceback = 2
 	curgp := mp.curg
-	casgstatus(curgp, _Grunning, _Gwaiting)
-	curgp.waitreason = waitReasonGarbageCollection
+	casGToWaiting(curgp, _Grunning, waitReasonGarbageCollection)
 
 	// Run gc on the g0 stack. We do this so that the g stack
 	// we're currently running on will no longer change. Cuts
@@ -1332,7 +1331,7 @@ func gcBgMarkWorker() {
 			// the G stack. However, stack shrinking is
 			// disabled for mark workers, so it is safe to
 			// read from the G stack.
-			casgstatus(gp, _Grunning, _Gwaiting)
+			casGToWaiting(gp, _Grunning, waitReasonGCWorkerActive)
 			switch pp.gcMarkWorkerMode {
 			default:
 				throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")

src/runtime/mgcmark.go

@@ -218,8 +218,7 @@ func markroot(gcw *gcWork, i uint32, flushBgCredit bool) int64 {
 			userG := getg().m.curg
 			selfScan := gp == userG && readgstatus(userG) == _Grunning
 			if selfScan {
-				casgstatus(userG, _Grunning, _Gwaiting)
-				userG.waitreason = waitReasonGarbageCollectionScan
+				casGToWaiting(userG, _Grunning, waitReasonGarbageCollectionScan)
 			}
 
 			// TODO: suspendG blocks (and spins) until gp
@@ -560,8 +559,7 @@ func gcAssistAlloc1(gp *g, scanWork int64) {
 	}
 
 	// gcDrainN requires the caller to be preemptible.
-	casgstatus(gp, _Grunning, _Gwaiting)
-	gp.waitreason = waitReasonGCAssistMarking
+	casGToWaiting(gp, _Grunning, waitReasonGCAssistMarking)
 
 	// drain own cached work first in the hopes that it
 	// will be more cache friendly.

src/runtime/proc.go

@@ -1027,6 +1027,14 @@ func casgstatus(gp *g, oldval, newval uint32) {
 	}
 }
 
+// casGToWaiting transitions gp from old to _Gwaiting, and sets the wait reason.
+//
+// Use this over casgstatus when possible to ensure that a waitreason is set.
+func casGToWaiting(gp *g, old uint32, reason waitReason) {
+	gp.waitreason = reason
+	casgstatus(gp, old, _Gwaiting)
+}
+
 // casgstatus(gp, oldstatus, Gcopystack), assuming oldstatus is Gwaiting or Grunnable.
 // Returns old status. Cannot call casgstatus directly, because we are racing with an
 // async wakeup that might come in from netpoll. If we see Gwaiting from the readgstatus,
@@ -1066,6 +1074,7 @@ func casGFromPreempted(gp *g, old, new uint32) bool {
 	if old != _Gpreempted || new != _Gwaiting {
 		throw("bad g transition")
 	}
+	gp.waitreason = waitReasonPreempted
 	return gp.atomicstatus.CompareAndSwap(_Gpreempted, _Gwaiting)
 }
 
@@ -1098,7 +1107,8 @@ func stopTheWorld(reason string) {
 		// must have preempted all goroutines, including any attempting
 		// to scan our stack, in which case, any stack shrinking will
 		// have already completed by the time we exit.
-		casgstatus(gp, _Grunning, _Gwaiting)
+		// Don't provide a wait reason because we're still executing.
+		casGToWaiting(gp, _Grunning, waitReasonStoppingTheWorld)
 		stopTheWorldWithSema()
 		casgstatus(gp, _Gwaiting, _Grunning)
 	})
@@ -3395,6 +3405,8 @@ func park_m(gp *g) {
 		traceGoPark(mp.waittraceev, mp.waittraceskip)
 	}
 
+	// N.B. Not using casGToWaiting here because the waitreason is
+	// set by park_m's caller.
 	casgstatus(gp, _Grunning, _Gwaiting)
 	dropg()
 
@@ -3468,7 +3480,6 @@ func preemptPark(gp *g) {
 		dumpgstatus(gp)
 		throw("bad g status")
 	}
-	gp.waitreason = waitReasonPreempted
 
 	if gp.asyncSafePoint {
 		// Double-check that async preemption does not
@@ -3545,7 +3556,7 @@ func goexit0(gp *g) {
 	gp._defer = nil // should be true already but just in case.
 	gp._panic = nil // non-nil for Goexit during panic. points at stack-allocated data.
 	gp.writebuf = nil
-	gp.waitreason = 0
+	gp.waitreason = waitReasonZero
 	gp.param = nil
 	gp.labels = nil
 	gp.timer = nil

src/runtime/runtime2.go

@@ -1060,8 +1060,11 @@ const (
 	waitReasonTraceReaderBlocked                      // "trace reader (blocked)"
 	waitReasonWaitForGCCycle                          // "wait for GC cycle"
 	waitReasonGCWorkerIdle                            // "GC worker (idle)"
+	waitReasonGCWorkerActive                          // "GC worker (active)"
 	waitReasonPreempted                               // "preempted"
 	waitReasonDebugCall                               // "debug call"
+	waitReasonGCMarkTermination                       // "GC mark termination"
+	waitReasonStoppingTheWorld                        // "stopping the world"
 )
 
 var waitReasonStrings = [...]string{
@@ -1092,8 +1095,11 @@ var waitReasonStrings = [...]string{
 	waitReasonTraceReaderBlocked:    "trace reader (blocked)",
 	waitReasonWaitForGCCycle:        "wait for GC cycle",
 	waitReasonGCWorkerIdle:          "GC worker (idle)",
+	waitReasonGCWorkerActive:        "GC worker (active)",
 	waitReasonPreempted:             "preempted",
 	waitReasonDebugCall:             "debug call",
+	waitReasonGCMarkTermination:     "GC mark termination",
+	waitReasonStoppingTheWorld:      "stopping the world",
 }
 
 func (w waitReason) String() string {