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>

1 files changed, 78 insertions, 13 deletions

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 18ce75d13a..a8b0cd76ac 100644
--- a/src/runtime/runtime2.go
+++ b/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
-	paniconfault   bool     // panic (instead of crash) on unexpected fault address
-	preemptscan    bool     // preempted g does scan for gc
-	gcscandone     bool     // g has scanned stack; protected by _Gscan bit in status
-	gcscanvalid    bool     // false at start of gc cycle, true if G has not run since last scan; TODO: remove?
-	throwsplit     bool     // must not split stack
-	raceignore     int8     // ignore race detection events
-	sysblocktraced bool     // StartTrace has emitted EvGoInSyscall about 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
+	waitsince      int64      // approx time when the g become blocked
+	waitreason     waitReason // if status==Gwaiting
+	preempt        bool       // preemption signal, duplicates stackguard0 = stackpreempt
+	paniconfault   bool       // panic (instead of crash) on unexpected fault address
+	preemptscan    bool       // preempted g does scan for gc
+	gcscandone     bool       // g has scanned stack; protected by _Gscan bit in status
+	gcscanvalid    bool       // false at start of gc cycle, true if G has not run since last scan; TODO: remove?
+	throwsplit     bool       // must not split stack
+	raceignore     int8       // ignore race detection events
+	sysblocktraced bool       // StartTrace has emitted EvGoInSyscall about 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