runtime: capture per-m trace state in a type

More tightening up of the tracer's interface.

While we're here, clarify why waittraceskip isn't included by explaining
what the wait* fields in the M are really for.

Change-Id: I0e7b4cac79fb77a7a0b3ca6b6cc267668e3610bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/494190
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: mknyszek

src/runtime/runtime2.go

@@ -577,13 +577,17 @@ type m struct {
 	lockedExt     uint32      // tracking for external LockOSThread
 	lockedInt     uint32      // tracking for internal lockOSThread
 	nextwaitm     muintptr    // next m waiting for lock
+
+	// wait* are used to carry arguments from gopark into park_m, because
+	// there's no stack to put them on. That is their sole purpose.
 	waitunlockf   func(*g, unsafe.Pointer) bool
 	waitlock      unsafe.Pointer
 	waittraceev   byte
 	waittraceskip int
-	startingtrace bool
-	syscalltick   uint32
-	freelink      *m // on sched.freem
+
+	syscalltick uint32
+	freelink    *m // on sched.freem
+	trace       mTraceState
 
 	// these are here because they are too large to be on the stack
 	// of low-level NOSPLIT functions.

src/runtime/trace.go

@@ -169,6 +169,11 @@ type gTraceState struct {
 	lastP          puintptr // last P emitted an event for this goroutine
 }
 
+// mTraceState is per-M state for the tracer.
+type mTraceState struct {
+	startingTrace bool // this M is in TraceStart, potentially before traceEnabled is true
+}
+
 // traceLockInit initializes global trace locks.
 func traceLockInit() {
 	lockInit(&trace.bufLock, lockRankTraceBuf)
@@ -252,10 +257,10 @@ func StartTrace() error {
 	// That would lead to an inconsistent trace:
 	// - either GoSysExit appears before EvGoInSyscall,
 	// - or GoSysExit appears for a goroutine for which we don't emit EvGoInSyscall below.
-	// To instruct traceEvent that it must not ignore events below, we set startingtrace.
+	// To instruct traceEvent that it must not ignore events below, we set trace.startingTrace.
 	// trace.enabled is set afterwards once we have emitted all preliminary events.
 	mp := getg().m
-	mp.startingtrace = true
+	mp.trace.startingTrace = true
 
 	// Obtain current stack ID to use in all traceEvGoCreate events below.
 	stkBuf := make([]uintptr, traceStackSize)
@@ -324,7 +329,7 @@ func StartTrace() error {
 	trace.strings = make(map[string]uint64)
 
 	trace.seqGC = 0
-	mp.startingtrace = false
+	mp.trace.startingTrace = false
 	trace.enabled = true
 
 	// Register runtime goroutine labels.
@@ -698,7 +703,7 @@ func traceEvent(ev byte, skip int, args ...uint64) {
 	// during tracing in exitsyscall is resolved by locking trace.bufLock in traceLockBuffer.
 	//
 	// Note trace_userTaskCreate runs the same check.
-	if !trace.enabled && !mp.startingtrace {
+	if !trace.enabled && !mp.trace.startingTrace {
 		traceReleaseBuffer(mp, pid)
 		return
 	}
@@ -1642,7 +1647,7 @@ func trace_userTaskCreate(id, parentID uint64, taskType string) {
 
 	// Same as in traceEvent.
 	mp, pid, bufp := traceAcquireBuffer()
-	if !trace.enabled && !mp.startingtrace {
+	if !trace.enabled && !mp.trace.startingTrace {
 		traceReleaseBuffer(mp, pid)
 		return
 	}
@@ -1664,7 +1669,7 @@ func trace_userRegion(id, mode uint64, name string) {
 	}
 
 	mp, pid, bufp := traceAcquireBuffer()
-	if !trace.enabled && !mp.startingtrace {
+	if !trace.enabled && !mp.trace.startingTrace {
 		traceReleaseBuffer(mp, pid)
 		return
 	}
@@ -1681,7 +1686,7 @@ func trace_userLog(id uint64, category, message string) {
 	}
 
 	mp, pid, bufp := traceAcquireBuffer()
-	if !trace.enabled && !mp.startingtrace {
+	if !trace.enabled && !mp.trace.startingTrace {
 		traceReleaseBuffer(mp, pid)
 		return
 	}