runtime: convert g.waitreason from string to uint8

Every time I poke at #14921, the g.waitreason string
pointer writes show up.

They're not particularly important performance-wise,
but it'd be nice to clear the noise away.

And it does open up a few extra bytes in the g struct
for some future use.

This is a re-roll of CL 99078, which was rolled
back because of failures on s390x.
Those failures were apparently due to an old version of gdb.

Change-Id: Icc2c12f449b2934063fd61e272e06237625ed589
Reviewed-on: https://go-review.googlesource.com/111256
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>

src/runtime/chan.go

@@ -142,7 +142,7 @@ func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
 		if !block {
 			return false
 		}
-		gopark(nil, nil, "chan send (nil chan)", traceEvGoStop, 2)
+		gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
 		throw("unreachable")
 	}
 
@@ -231,7 +231,7 @@ func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
 	gp.waiting = mysg
 	gp.param = nil
 	c.sendq.enqueue(mysg)
-	goparkunlock(&c.lock, "chan send", traceEvGoBlockSend, 3)
+	goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)
 
 	// someone woke us up.
 	if mysg != gp.waiting {
@@ -426,7 +426,7 @@ func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool)
 		if !block {
 			return
 		}
-		gopark(nil, nil, "chan receive (nil chan)", traceEvGoStop, 2)
+		gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
 		throw("unreachable")
 	}
 
@@ -517,7 +517,7 @@ func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool)
 	mysg.c = c
 	gp.param = nil
 	c.recvq.enqueue(mysg)
-	goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv, 3)
+	goparkunlock(&c.lock, waitReasonChanReceive, traceEvGoBlockRecv, 3)
 
 	// someone woke us up
 	if mysg != gp.waiting {

src/runtime/heapdump.go

@@ -349,7 +349,7 @@ func dumpgoroutine(gp *g) {
 	dumpbool(isSystemGoroutine(gp))
 	dumpbool(false) // isbackground
 	dumpint(uint64(gp.waitsince))
-	dumpstr(gp.waitreason)
+	dumpstr(gp.waitreason.String())
 	dumpint(uint64(uintptr(gp.sched.ctxt)))
 	dumpint(uint64(uintptr(unsafe.Pointer(gp.m))))
 	dumpint(uint64(uintptr(unsafe.Pointer(gp._defer))))
@@ -658,7 +658,7 @@ func mdump() {
 func writeheapdump_m(fd uintptr) {
 	_g_ := getg()
 	casgstatus(_g_.m.curg, _Grunning, _Gwaiting)
-	_g_.waitreason = "dumping heap"
+	_g_.waitreason = waitReasonDumpingHeap
 
 	// Update stats so we can dump them.
 	// As a side effect, flushes all the MCaches so the MSpan.freelist

src/runtime/mfinal.go

@@ -172,7 +172,7 @@ func runfinq() {
 			gp := getg()
 			fing = gp
 			fingwait = true
-			goparkunlock(&finlock, "finalizer wait", traceEvGoBlock, 1)
+			goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
 			continue
 		}
 		unlock(&finlock)

src/runtime/mgc.go

@@ -1149,7 +1149,7 @@ func gcWaitOnMark(n uint32) {
 		gp := getg()
 		gp.schedlink = work.sweepWaiters.head
 		work.sweepWaiters.head.set(gp)
-		goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1)
+		goparkunlock(&work.sweepWaiters.lock, waitReasonWaitForGCCycle, traceEvGoBlock, 1)
 	}
 }
 
@@ -1527,7 +1527,7 @@ func gcMarkTermination(nextTriggerRatio float64) {
 	_g_.m.traceback = 2
 	gp := _g_.m.curg
 	casgstatus(gp, _Grunning, _Gwaiting)
-	gp.waitreason = "garbage collection"
+	gp.waitreason = 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
@@ -1796,7 +1796,7 @@ func gcBgMarkWorker(_p_ *p) {
 				}
 			}
 			return true
-		}, unsafe.Pointer(park), "GC worker (idle)", traceEvGoBlock, 0)
+		}, unsafe.Pointer(park), waitReasonGCWorkerIdle, traceEvGoBlock, 0)
 
 		// Loop until the P dies and disassociates this
 		// worker (the P may later be reused, in which case

src/runtime/mgcmark.go

@@ -251,7 +251,7 @@ func markroot(gcw *gcWork, i uint32) {
 			selfScan := gp == userG && readgstatus(userG) == _Grunning
 			if selfScan {
 				casgstatus(userG, _Grunning, _Gwaiting)
-				userG.waitreason = "garbage collection scan"
+				userG.waitreason = waitReasonGarbageCollectionScan
 			}
 
 			// TODO: scang blocks until gp's stack has
@@ -549,7 +549,7 @@ func gcAssistAlloc1(gp *g, scanWork int64) {
 
 	// gcDrainN requires the caller to be preemptible.
 	casgstatus(gp, _Grunning, _Gwaiting)
-	gp.waitreason = "GC assist marking"
+	gp.waitreason = waitReasonGCAssistMarking
 
 	// drain own cached work first in the hopes that it
 	// will be more cache friendly.
@@ -648,7 +648,7 @@ func gcParkAssist() bool {
 		return false
 	}
 	// Park.
-	goparkunlock(&work.assistQueue.lock, "GC assist wait", traceEvGoBlockGC, 2)
+	goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceEvGoBlockGC, 2)
 	return true
 }
 

src/runtime/mgcsweep.go

@@ -49,7 +49,7 @@ func bgsweep(c chan int) {
 	lock(&sweep.lock)
 	sweep.parked = true
 	c <- 1
-	goparkunlock(&sweep.lock, "GC sweep wait", traceEvGoBlock, 1)
+	goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)
 
 	for {
 		for gosweepone() != ^uintptr(0) {
@@ -68,7 +68,7 @@ func bgsweep(c chan int) {
 			continue
 		}
 		sweep.parked = true
-		goparkunlock(&sweep.lock, "GC sweep wait", traceEvGoBlock, 1)
+		goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)
 	}
 }
 

src/runtime/netpoll.go

@@ -363,7 +363,7 @@ func netpollblock(pd *pollDesc, mode int32, waitio bool) bool {
 	// this is necessary because runtime_pollUnblock/runtime_pollSetDeadline/deadlineimpl
 	// do the opposite: store to closing/rd/wd, membarrier, load of rg/wg
 	if waitio || netpollcheckerr(pd, mode) == 0 {
-		gopark(netpollblockcommit, unsafe.Pointer(gpp), "IO wait", traceEvGoBlockNet, 5)
+		gopark(netpollblockcommit, unsafe.Pointer(gpp), waitReasonIOWait, traceEvGoBlockNet, 5)
 	}
 	// be careful to not lose concurrent READY notification
 	old := atomic.Xchguintptr(gpp, 0)

src/runtime/proc.go

@@ -214,7 +214,7 @@ func main() {
 		}
 	}
 	if atomic.Load(&panicking) != 0 {
-		gopark(nil, nil, "panicwait", traceEvGoStop, 1)
+		gopark(nil, nil, waitReasonPanicWait, traceEvGoStop, 1)
 	}
 
 	exit(0)
@@ -245,7 +245,7 @@ func forcegchelper() {
 			throw("forcegc: phase error")
 		}
 		atomic.Store(&forcegc.idle, 1)
-		goparkunlock(&forcegc.lock, "force gc (idle)", traceEvGoBlock, 1)
+		goparkunlock(&forcegc.lock, waitReasonForceGGIdle, traceEvGoBlock, 1)
 		// this goroutine is explicitly resumed by sysmon
 		if debug.gctrace > 0 {
 			println("GC forced")
@@ -274,7 +274,11 @@ func goschedguarded() {
 // If unlockf returns false, the goroutine is resumed.
 // unlockf must not access this G's stack, as it may be moved between
 // the call to gopark and the call to unlockf.
-func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason string, traceEv byte, traceskip int) {
+// Reason explains why the goroutine has been parked.
+// It is displayed in stack traces and heap dumps.
+// Reasons should be unique and descriptive.
+// Do not re-use reasons, add new ones.
+func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceEv byte, traceskip int) {
 	mp := acquirem()
 	gp := mp.curg
 	status := readgstatus(gp)
@@ -293,7 +297,7 @@ func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason s
 
 // Puts the current goroutine into a waiting state and unlocks the lock.
 // The goroutine can be made runnable again by calling goready(gp).
-func goparkunlock(lock *mutex, reason string, traceEv byte, traceskip int) {
+func goparkunlock(lock *mutex, reason waitReason, traceEv byte, traceskip int) {
 	gopark(parkunlock_c, unsafe.Pointer(lock), reason, traceEv, traceskip)
 }
 
@@ -2676,7 +2680,7 @@ 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 = ""
+	gp.waitreason = 0
 	gp.param = nil
 	gp.labels = nil
 	gp.timer = nil
@@ -4542,7 +4546,7 @@ func schedtrace(detailed bool) {
 		if lockedm != nil {
 			id2 = lockedm.id
 		}
-		print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason, ") m=", id1, " lockedm=", id2, "\n")
+		print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason.String(), ") m=", id1, " lockedm=", id2, "\n")
 	}
 	unlock(&allglock)
 	unlock(&sched.lock)

src/runtime/runtime2.go

@@ -358,20 +358,20 @@ type g struct {
 	atomicstatus   uint32
 	stackLock      uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
 	goid           int64
-	waitsince      int64  // approx time when the g become blocked
-	waitreason     string // if status==Gwaiting
 	schedlink      guintptr
-	preempt        bool     // preemption signal, duplicates stackguard0 = stackpreempt
+	waitsince      int64      // approx time when the g become blocked
-	paniconfault   bool     // panic (instead of crash) on unexpected fault address
+	waitreason     waitReason // if status==Gwaiting
-	preemptscan    bool     // preempted g does scan for gc
+	preempt        bool       // preemption signal, duplicates stackguard0 = stackpreempt
-	gcscandone     bool     // g has scanned stack; protected by _Gscan bit in status
+	paniconfault   bool       // panic (instead of crash) on unexpected fault address
-	gcscanvalid    bool     // false at start of gc cycle, true if G has not run since last scan; TODO: remove?
+	preemptscan    bool       // preempted g does scan for gc
-	throwsplit     bool     // must not split stack
+	gcscandone     bool       // g has scanned stack; protected by _Gscan bit in status
-	raceignore     int8     // ignore race detection events
+	gcscanvalid    bool       // false at start of gc cycle, true if G has not run since last scan; TODO: remove?
-	sysblocktraced bool     // StartTrace has emitted EvGoInSyscall about this goroutine
+	throwsplit     bool       // must not split stack
-	sysexitticks   int64    // cputicks when syscall has returned (for tracing)
+	raceignore     int8       // ignore race detection events
-	traceseq       uint64   // trace event sequencer
+	sysblocktraced bool       // StartTrace has emitted EvGoInSyscall about this goroutine
-	tracelastp     puintptr // last P emitted an event for this goroutine
+	sysexitticks   int64      // cputicks when syscall has returned (for tracing)
+	traceseq       uint64     // trace event sequencer
+	tracelastp     puintptr   // last P emitted an event for this goroutine
 	lockedm        muintptr
 	sig            uint32
 	writebuf       []byte
@@ -756,6 +756,71 @@ const (
 // The maximum number of frames we print for a traceback
 const _TracebackMaxFrames = 100
 
+// A waitReason explains why a goroutine has been stopped.
+// See gopark. Do not re-use waitReasons, add new ones.
+type waitReason uint8
+
+const (
+	waitReasonZero                  waitReason = iota // ""
+	waitReasonGCAssistMarking                         // "GC assist marking"
+	waitReasonIOWait                                  // "IO wait"
+	waitReasonChanReceiveNilChan                      // "chan receive (nil chan)"
+	waitReasonChanSendNilChan                         // "chan send (nil chan)"
+	waitReasonDumpingHeap                             // "dumping heap"
+	waitReasonGarbageCollection                       // "garbage collection"
+	waitReasonGarbageCollectionScan                   // "garbage collection scan"
+	waitReasonPanicWait                               // "panicwait"
+	waitReasonSelect                                  // "select"
+	waitReasonSelectNoCases                           // "select (no cases)"
+	waitReasonGCAssistWait                            // "GC assist wait"
+	waitReasonGCSweepWait                             // "GC sweep wait"
+	waitReasonChanReceive                             // "chan receive"
+	waitReasonChanSend                                // "chan send"
+	waitReasonFinalizerWait                           // "finalizer wait"
+	waitReasonForceGGIdle                             // "force gc (idle)"
+	waitReasonSemacquire                              // "semacquire"
+	waitReasonSleep                                   // "sleep"
+	waitReasonSyncCondWait                            // "sync.Cond.Wait"
+	waitReasonTimerGoroutineIdle                      // "timer goroutine (idle)"
+	waitReasonTraceReaderBlocked                      // "trace reader (blocked)"
+	waitReasonWaitForGCCycle                          // "wait for GC cycle"
+	waitReasonGCWorkerIdle                            // "GC worker (idle)"
+)
+
+var waitReasonStrings = [...]string{
+	waitReasonZero:                  "",
+	waitReasonGCAssistMarking:       "GC assist marking",
+	waitReasonIOWait:                "IO wait",
+	waitReasonChanReceiveNilChan:    "chan receive (nil chan)",
+	waitReasonChanSendNilChan:       "chan send (nil chan)",
+	waitReasonDumpingHeap:           "dumping heap",
+	waitReasonGarbageCollection:     "garbage collection",
+	waitReasonGarbageCollectionScan: "garbage collection scan",
+	waitReasonPanicWait:             "panicwait",
+	waitReasonSelect:                "select",
+	waitReasonSelectNoCases:         "select (no cases)",
+	waitReasonGCAssistWait:          "GC assist wait",
+	waitReasonGCSweepWait:           "GC sweep wait",
+	waitReasonChanReceive:           "chan receive",
+	waitReasonChanSend:              "chan send",
+	waitReasonFinalizerWait:         "finalizer wait",
+	waitReasonForceGGIdle:           "force gc (idle)",
+	waitReasonSemacquire:            "semacquire",
+	waitReasonSleep:                 "sleep",
+	waitReasonSyncCondWait:          "sync.Cond.Wait",
+	waitReasonTimerGoroutineIdle:    "timer goroutine (idle)",
+	waitReasonTraceReaderBlocked:    "trace reader (blocked)",
+	waitReasonWaitForGCCycle:        "wait for GC cycle",
+	waitReasonGCWorkerIdle:          "GC worker (idle)",
+}
+
+func (w waitReason) String() string {
+	if w < 0 || w >= waitReason(len(waitReasonStrings)) {
+		return "unknown wait reason"
+	}
+	return waitReasonStrings[w]
+}
+
 var (
 	allglen    uintptr
 	allm       *m

src/runtime/select.go

@@ -99,7 +99,7 @@ func selparkcommit(gp *g, _ unsafe.Pointer) bool {
 }
 
 func block() {
-	gopark(nil, nil, "select (no cases)", traceEvGoStop, 1) // forever
+	gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
 }
 
 // selectgo implements the select statement.
@@ -309,7 +309,7 @@ loop:
 
 	// wait for someone to wake us up
 	gp.param = nil
-	gopark(selparkcommit, nil, "select", traceEvGoBlockSelect, 1)
+	gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
 
 	sellock(scases, lockorder)
 

src/runtime/sema.go

@@ -141,7 +141,7 @@ func semacquire1(addr *uint32, lifo bool, profile semaProfileFlags) {
 		// Any semrelease after the cansemacquire knows we're waiting
 		// (we set nwait above), so go to sleep.
 		root.queue(addr, s, lifo)
-		goparkunlock(&root.lock, "semacquire", traceEvGoBlockSync, 4)
+		goparkunlock(&root.lock, waitReasonSemacquire, traceEvGoBlockSync, 4)
 		if s.ticket != 0 || cansemacquire(addr) {
 			break
 		}
@@ -507,7 +507,7 @@ func notifyListWait(l *notifyList, t uint32) {
 		l.tail.next = s
 	}
 	l.tail = s
-	goparkunlock(&l.lock, "sync.Cond.Wait", traceEvGoBlockCond, 3)
+	goparkunlock(&l.lock, waitReasonSyncCondWait, traceEvGoBlockCond, 3)
 	if t0 != 0 {
 		blockevent(s.releasetime-t0, 2)
 	}

src/runtime/sizeof_test.go

@@ -23,7 +23,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{runtime.G{}, 220, 384}, // g, but exported for testing
+		{runtime.G{}, 216, 376}, // g, but exported for testing
 	}
 
 	for _, tt := range tests {

src/runtime/time.go

@@ -99,7 +99,7 @@ func timeSleep(ns int64) {
 	tb := t.assignBucket()
 	lock(&tb.lock)
 	tb.addtimerLocked(t)
-	goparkunlock(&tb.lock, "sleep", traceEvGoSleep, 2)
+	goparkunlock(&tb.lock, waitReasonSleep, traceEvGoSleep, 2)
 }
 
 // startTimer adds t to the timer heap.
@@ -250,7 +250,7 @@ func timerproc(tb *timersBucket) {
 		if delta < 0 || faketime > 0 {
 			// No timers left - put goroutine to sleep.
 			tb.rescheduling = true
-			goparkunlock(&tb.lock, "timer goroutine (idle)", traceEvGoBlock, 1)
+			goparkunlock(&tb.lock, waitReasonTimerGoroutineIdle, traceEvGoBlock, 1)
 			continue
 		}
 		// At least one timer pending. Sleep until then.

src/runtime/trace.go

@@ -392,7 +392,7 @@ func ReadTrace() []byte {
 	// Wait for new data.
 	if trace.fullHead == 0 && !trace.shutdown {
 		trace.reader.set(getg())
-		goparkunlock(&trace.lock, "trace reader (blocked)", traceEvGoBlock, 2)
+		goparkunlock(&trace.lock, waitReasonTraceReaderBlocked, traceEvGoBlock, 2)
 		lock(&trace.lock)
 	}
 	// Write a buffer.

src/runtime/traceback.go

@@ -872,8 +872,8 @@ func goroutineheader(gp *g) {
 	}
 
 	// Override.
-	if gpstatus == _Gwaiting && gp.waitreason != "" {
+	if gpstatus == _Gwaiting && gp.waitreason != waitReasonZero {
-		status = gp.waitreason
+		status = gp.waitreason.String()
 	}
 
 	// approx time the G is blocked, in minutes