runtime: schedule fractional workers on all Ps

Currently only a single P can run a fractional mark worker at a time.
This doesn't let us spread out the load, so it gets concentrated on
whatever unlucky P picks up the token to run a fractional worker. This
can significantly delay goroutines on that P.

This commit changes this scheduling rule so each P separately
schedules fractional workers. This can significantly reduce the load
on any individual P and allows workers to self-preempt earlier. It
does have the downside that it's possible for all Ps to be in
fractional workers simultaneously (an effect STW).

Updates #21698.

Change-Id: Ia1e300c422043fa62bb4e3dd23c6232d81e4419c
Reviewed-on: https://go-review.googlesource.com/68574
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/runtime2.go

@@ -522,9 +522,10 @@ type p struct {
 	palloc persistentAlloc // per-P to avoid mutex
 
 	// Per-P GC state
-	gcAssistTime     int64 // Nanoseconds in assistAlloc
-	gcBgMarkWorker   guintptr
-	gcMarkWorkerMode gcMarkWorkerMode
+	gcAssistTime         int64 // Nanoseconds in assistAlloc
+	gcFractionalMarkTime int64 // Nanoseconds in fractional mark worker
+	gcBgMarkWorker       guintptr
+	gcMarkWorkerMode     gcMarkWorkerMode
 
 	// gcMarkWorkerStartTime is the nanotime() at which this mark
 	// worker started.