Revert "runtime: disable use of runnext on wasm"

This reverts CL 557437.

Reason for revert: Appears to have broken wasip1 builders.

For golang#65178.

Change-Id: I59c1a310eb56589c768536fe444c1efaf862f8b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/559237
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/net/net_fake.go

@@ -14,6 +14,7 @@ import (
 	"errors"
 	"io"
 	"os"
+	"runtime"
 	"sync"
 	"sync/atomic"
 	"syscall"
@@ -516,6 +517,14 @@ func (pq *packetQueue) send(dt *deadlineTimer, b []byte, from sockaddr, block bo
 		full = pq.full
 	}
 
+	// Before we check dt.expired, yield to other goroutines.
+	// This may help to prevent starvation of the goroutine that runs the
+	// deadlineTimer's time.After callback.
+	//
+	// TODO(#65178): Remove this when the runtime scheduler no longer starves
+	// runnable goroutines.
+	runtime.Gosched()
+
 	select {
 	case <-dt.expired:
 		return 0, os.ErrDeadlineExceeded
@@ -567,6 +576,14 @@ func (pq *packetQueue) recvfrom(dt *deadlineTimer, b []byte, wholePacket bool, c
 		empty = pq.empty
 	}
 
+	// Before we check dt.expired, yield to other goroutines.
+	// This may help to prevent starvation of the goroutine that runs the
+	// deadlineTimer's time.After callback.
+	//
+	// TODO(#65178): Remove this when the runtime scheduler no longer starves
+	// runnable goroutines.
+	runtime.Gosched()
+
 	select {
 	case <-dt.expired:
 		return 0, nil, os.ErrDeadlineExceeded

src/runtime/proc.go

@@ -167,7 +167,7 @@ func main() {
 	// Allow newproc to start new Ms.
 	mainStarted = true
 
-	if haveSysmon {
+	if GOARCH != "wasm" { // no threads on wasm yet, so no sysmon
 		systemstack(func() {
 			newm(sysmon, nil, -1)
 		})
@@ -5933,11 +5933,6 @@ var forcegcperiod int64 = 2 * 60 * 1e9
 // golang.org/issue/42515 is needed on NetBSD.
 var needSysmonWorkaround bool = false
 
-// haveSysmon indicates whether there is sysmon thread support.
-//
-// No threads on wasm yet, so no sysmon.
-const haveSysmon = GOARCH != "wasm"
-
 // Always runs without a P, so write barriers are not allowed.
 //
 //go:nowritebarrierrec
@@ -6118,10 +6113,7 @@ func retake(now int64) uint32 {
 		s := pp.status
 		sysretake := false
 		if s == _Prunning || s == _Psyscall {
-			// Preempt G if it's running on the same schedtick for
-			// too long. This could be from a single long-running
-			// goroutine or a sequence of goroutines run via
-			// runnext, which share a single schedtick time slice.
+			// Preempt G if it's running for too long.
 			t := int64(pp.schedtick)
 			if int64(pd.schedtick) != t {
 				pd.schedtick = uint32(t)
@@ -6632,17 +6624,6 @@ const randomizeScheduler = raceenabled
 // If the run queue is full, runnext puts g on the global queue.
 // Executed only by the owner P.
 func runqput(pp *p, gp *g, next bool) {
-	if !haveSysmon && next {
-		// A runnext goroutine shares the same time slice as the
-		// current goroutine (inheritTime from runqget). To prevent a
-		// ping-pong pair of goroutines from starving all others, we
-		// depend on sysmon to preempt "long-running goroutines". That
-		// is, any set of goroutines sharing the same time slice.
-		//
-		// If there is no sysmon, we must avoid runnext entirely or
-		// risk starvation.
-		next = false
-	}
 	if randomizeScheduler && next && randn(2) == 0 {
 		next = false
 	}

src/time/sleep_test.go

@@ -96,6 +96,11 @@ func TestAfterFuncStarvation(t *testing.T) {
 	// the AfterFunc goroutine instead of the runnable channel goroutine.
 	// However, in https://go.dev/issue/65178 this was observed to live-lock
 	// on wasip1/wasm and js/wasm after <10000 runs.
+
+	if runtime.GOARCH == "wasm" {
+		testenv.SkipFlaky(t, 65178)
+	}
+
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
 
 	var (