runtime: don't track scheduling latency for _Grunning <-> _Gsyscall

The current logic causes much more tracking than necessary, when really
_Grunning and _Gsyscall are both sort of "running" from the perspective
of tracking scheduling latency.

This makes cgo calls and syscalls a little faster in the single-threaded
case, and shows much larger improvement in the multi-threaded case
by removing updates of shared variables (though this parallel
microbenchmark is a little unrealistic, so don't ascribe too much weight
to it).

goos: linux
goarch: amd64
pkg: internal/runtime/cgobench
cpu: AMD EPYC 7B13
                   │  after.out  │            after-2.out             │
                   │   sec/op    │   sec/op     vs base               │
CgoCall-64           35.83n ± 1%   34.69n ± 1%   -3.20% (p=0.002 n=6)
CgoCallParallel-64   5.338n ± 1%   1.352n ± 4%  -74.67% (p=0.002 n=6)

Change-Id: I2ea494dd5ebbbfb457373549986fbe2fbe318d45
Reviewed-on: https://go-review.googlesource.com/c/go/+/646275
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/proc.go

@@ -1316,8 +1316,9 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		})
 	}
 
-	if oldval == _Grunning {
-		// Track every gTrackingPeriod time a goroutine transitions out of running.
+	if (oldval == _Grunning || oldval == _Gsyscall) && (newval != _Grunning && newval != _Gsyscall) {
+		// Track every gTrackingPeriod time a goroutine transitions out of _Grunning or _Gsyscall.
+		// Do not track _Grunning <-> _Gsyscall transitions, since they're two very similar states.
 		if casgstatusAlwaysTrack || gp.trackingSeq%gTrackingPeriod == 0 {
 			gp.tracking = true
 		}