runtime: manage gcBgMarkWorkers with a global pool

Background mark workers perform per-P marking work. Currently each
worker is assigned a P at creation time. The worker "attaches" to the P
via p.gcBgMarkWorker, making itself (usually) available to
findRunnableGCWorker for scheduling GC work.

While running gcMarkDone, the worker "detaches" from the P (by clearing
p.gcBgMarkWorker), since it may park for other reasons and should not be
scheduled by findRunnableGCWorker.

Unfortunately, this design is complex and difficult to reason about. We
simplify things by changing the design to eliminate the hard P
attachment. Rather than workers always performing work from the same P,
workers perform work for whichever P they find themselves on. On park,
the workers are placed in a pool of free workers, which each P's
findRunnableGCWorker can use to run a worker for its P.

Now if a worker parks in gcMarkDone, a P may simply use another worker
from the pool to complete its own work.

The P's GC worker mode is used to communicate the mode to run to the
selected worker. It is also used to emit the appropriate worker
EvGoStart tracepoint. This is a slight change, as this G may be
preempted (e.g., in gcMarkDone). When it is rescheduled, the trace
viewer will show it as a normal goroutine again. It is currently a bit
difficult to connect to the original worker tracepoint, as the viewer
does not display the goid for the original worker (though the data is in
the trace file).

Change-Id: Id7bd3a364dc18a4d2b1c99c4dc4810fae1293c1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/262348
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Trust: Michael Pratt <mpratt@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/runtime/trace.go b/src/runtime/trace.go
index d3ecd148be..bcd0b9d56c 100644
--- a/src/runtime/trace.go
+++ b/src/runtime/trace.go
@@ -1064,7 +1064,7 @@ func traceGoStart() {
 	_g_ := getg().m.curg
 	_p_ := _g_.m.p
 	_g_.traceseq++
-	if _g_ == _p_.ptr().gcBgMarkWorker.ptr() {
+	if _p_.ptr().gcMarkWorkerMode != gcMarkWorkerNotWorker {
 		traceEvent(traceEvGoStartLabel, -1, uint64(_g_.goid), _g_.traceseq, trace.markWorkerLabels[_p_.ptr().gcMarkWorkerMode])
 	} else if _g_.tracelastp == _p_ {
 		traceEvent(traceEvGoStartLocal, -1, uint64(_g_.goid))