Revert "runtime: implement runqdrain() for GC mark worker goroutines"

This reverts CL 310149.

Reason for revert: Breaks longtest builders:

https://build.golang.org/log/6af9fb147fa3101154db10e7ce055e8267cd4c93
https://build.golang.org/log/172ed6e1ec3bb503370333ee421c590fd2a72d0a

Change-Id: Iaf5a8b9eec51d0517311e050d0b0f7569759d292
Reviewed-on: https://go-review.googlesource.com/c/go/+/312129
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/runtime/mgc.go

@@ -1273,11 +1273,15 @@ func gcBgMarkWorker() {
 					// everything out of the run
 					// queue so it can run
 					// somewhere else.
-					if drainQ, n := runqdrain(pp); n > 0 {
 					lock(&sched.lock)
-						globrunqputbatch(&drainQ, int32(n))
+					for {
-						unlock(&sched.lock)
+						gp, _ := runqget(pp)
+						if gp == nil {
+							break
 						}
+						globrunqput(gp)
+					}
+					unlock(&sched.lock)
 				}
 				// Go back to draining, this time
 				// without preemption.

src/runtime/proc.go

@@ -5954,44 +5954,6 @@ func runqget(_p_ *p) (gp *g, inheritTime bool) {
 	}
 }
 
-// runqdrain drains the local runnable queue of _p_ and returns all g's in it.
-// Executed only by the owner P.
-func runqdrain(_p_ *p) (drainQ gQueue, n uint32) {
-	var getNext bool
-	oldNext := _p_.runnext
-	if oldNext != 0 && _p_.runnext.cas(oldNext, 0) {
-		drainQ.pushBack(oldNext.ptr())
-		n++
-		getNext = true
-	}
-
-	for {
-		h := atomic.LoadAcq(&_p_.runqhead) // load-acquire, synchronize with other consumers
-		t := _p_.runqtail
-		qn := t - h
-		if qn == 0 {
-			return
-		}
-		for i := uint32(0); i < qn; i++ {
-			gp := _p_.runq[(h+i)%uint32(len(_p_.runq))].ptr()
-			drainQ.pushBack(gp)
-		}
-		if atomic.CasRel(&_p_.runqhead, h, h+qn) { // cas-release, commits consume
-			n += qn
-			return
-		}
-
-		// Clean up if it failed to drain _p_ in this round and start over until it succeed.
-		drainQ = gQueue{}
-		n = 0
-		// Push the prior old _p_.runnext back into drainQ.
-		if getNext {
-			drainQ.pushBack(oldNext.ptr())
-			n++
-		}
-	}
-}
-
 // Grabs a batch of goroutines from _p_'s runnable queue into batch.
 // Batch is a ring buffer starting at batchHead.
 // Returns number of grabbed goroutines.

src/runtime/runtime2.go

@@ -622,9 +622,6 @@ type p struct {
 	// unit and eliminates the (potentially large) scheduling
 	// latency that otherwise arises from adding the ready'd
 	// goroutines to the end of the run queue.
-	//
-	// Note that while other P's may atomically CAS this to zero,
-	// only the owner P can CAS it to a valid G.
 	runnext guintptr
 
 	// Available G's (status == Gdead)