runtime: disable idle mark workers with at least one dedicated worker

This change completes the proposal laid out in #44163. With #44313
resolved, we now ensure that stopped Ms are able to wake up and become
dedicated GC workers. As a result, idle GC workers are in theory no
longer required to be a proxy for scheduling dedicated mark workers.

And, with at least one dedicated mark worker running (which is
non-preemptible) we ensure the GC makes progress in all circumstances
when at least one is running. Currently we ensure at least one idle mark
worker is available at all times because it's possible before #44313
that a dedicated worker doesn't ever get scheduled, leading to a
deadlock if user goroutines block on a GC completing. But now that extra
idle mark worker should be unnecessary to ensure GC progress when at
least one dedicated mark worker is going to be scheduled.

Fixes #44163.

Change-Id: I62889ef2db4e69d44da883e8e6eebcfe5398c86d
Reviewed-on: https://go-review.googlesource.com/c/go/+/395634
Reviewed-by: Michael Pratt <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: mknyszek

src/runtime/mgcpacer.go

@@ -287,7 +287,7 @@ type gcControllerState struct {
 	//
 	// The top int32 is the maximum number of idle mark workers allowed to
 	// execute concurrently. Normally, this number is just gomaxprocs. However,
-	// during periodic GC cycles it is set to 1 because the system is idle
+	// during periodic GC cycles it is set to 0 because the system is idle
 	// anyway; there's no need to go full blast on all of GOMAXPROCS.
 	//
 	// The maximum number of idle mark workers is used to prevent new workers
@@ -296,17 +296,22 @@ type gcControllerState struct {
 	// transiently exceed the maximum. This could happen if the maximum changes
 	// just after a GC ends, and an M with no P.
 	//
-	// Note that the maximum may not be zero because idle-priority mark workers
-	// are vital to GC progress. Consider a situation in which goroutines
-	// block on the GC (such as via runtime.GOMAXPROCS) and only fractional
-	// mark workers are scheduled (e.g. GOMAXPROCS=1). Without idle-priority
-	// mark workers, the last running M might skip scheduling a fractional
-	// mark worker if its utilization goal is met, such that once it goes to
-	// sleep (because there's nothing to do), there will be nothing else to
-	// spin up a new M for the fractional worker in the future, stalling GC
-	// progress and causing a deadlock. However, idle-priority workers will
-	// *always* run when there is nothing left to do, ensuring the GC makes
-	// progress.
+	// Note that if we have no dedicated mark workers, we set this value to
+	// 1 in this case we only have fractional GC workers which aren't scheduled
+	// strictly enough to ensure GC progress. As a result, idle-priority mark
+	// workers are vital to GC progress in these situations.
+	//
+	// For example, consider a situation in which goroutines block on the GC
+	// (such as via runtime.GOMAXPROCS) and only fractional mark workers are
+	// scheduled (e.g. GOMAXPROCS=1). Without idle-priority mark workers, the
+	// last running M might skip scheduling a fractional mark worker if its
+	// utilization goal is met, such that once it goes to sleep (because there's
+	// nothing to do), there will be nothing else to spin up a new M for the
+	// fractional worker in the future, stalling GC progress and causing a
+	// deadlock. However, idle-priority workers will *always* run when there is
+	// nothing left to do, ensuring the GC makes progress.
+	//
+	// See github.com/golang/go/issues/44163 for more details.
 	idleMarkWorkers atomic.Uint64
 
 	// assistWorkPerByte is the ratio of scan work to allocated
@@ -430,11 +435,19 @@ func (c *gcControllerState) startCycle(markStartTime int64, procs int, trigger g
 	}
 
 	if trigger.kind == gcTriggerTime {
-		// During a periodic GC cycle, avoid having more than
-		// one idle mark worker running at a time. We need to have
-		// at least one to ensure the GC makes progress, but more than
-		// one is unnecessary.
-		c.setMaxIdleMarkWorkers(1)
+		// During a periodic GC cycle, reduce the number of idle mark workers
+		// required. However, we need at least one dedicated mark worker or
+		// idle GC worker to ensure GC progress in some scenarios (see comment
+		// on maxIdleMarkWorkers).
+		if c.dedicatedMarkWorkersNeeded > 0 {
+			c.setMaxIdleMarkWorkers(0)
+		} else {
+			// TODO(mknyszek): The fundamental reason why we need this is because
+			// we can't count on the fractional mark worker to get scheduled.
+			// Fix that by ensuring it gets scheduled according to its quota even
+			// if the rest of the application is idle.
+			c.setMaxIdleMarkWorkers(1)
+		}
 	} else {
 		// N.B. gomaxprocs and dedicatedMarkWorkersNeeded is guaranteed not to
 		// change during a GC cycle.