runtime: factor our oneNewExtraM trace code

In the interest of further cleaning up the trace.go API, move the trace
logic in oneNewExtraM into its own function.

Change-Id: I5cf478cb8cd0d301ee3b068347ed48ce768b8882
Reviewed-on: https://go-review.googlesource.com/c/go/+/494186
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/proc.go

@@ -2004,12 +2004,7 @@ func oneNewExtraM() {
 		gp.racectx = racegostart(abi.FuncPCABIInternal(newextram) + sys.PCQuantum)
 	}
 	if traceEnabled() {
-		// Trigger two trace events for the locked g in the extra m,
+		traceOneNewExtraM(gp)
-		// since the next event of the g will be traceEvGoSysExit in exitsyscall,
-		// while calling from C thread to Go.
-		traceGoCreate(gp, 0) // no start pc
-		gp.traceseq++
-		traceEvent(traceEvGoInSyscall, -1, gp.goid)
 	}
 	// put on allg for garbage collector
 	allgadd(gp)

src/runtime/trace.go

@@ -1713,3 +1713,16 @@ func startPCforTrace(pc uintptr) uintptr {
 	}
 	return f.datap.textAddr(*(*uint32)(w))
 }
+
+// traceOneNewExtraM registers the fact that a new extra M was created with
+// the tracer. This matters if the M (which has an attached G) is used while
+// the trace is still active because if it is, we need the fact that it exists
+// to show up in the final trace.
+func traceOneNewExtraM(gp *g) {
+	// Trigger two trace events for the locked g in the extra m,
+	// since the next event of the g will be traceEvGoSysExit in exitsyscall,
+	// while calling from C thread to Go.
+	traceGoCreate(gp, 0) // no start pc
+	gp.traceseq++
+	traceEvent(traceEvGoInSyscall, -1, gp.goid)
+}