1 files changed, 21 insertions, 2 deletions
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index c1b3ce20d5..410bd01169 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -167,7 +167,7 @@ func main() {
 	// Allow newproc to start new Ms.
 	mainStarted = true
 
-	if GOARCH != "wasm" { // no threads on wasm yet, so no sysmon
+	if haveSysmon {
 		systemstack(func() {
 			newm(sysmon, nil, -1)
 		})
@@ -5933,6 +5933,11 @@ 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
@@ -6113,7 +6118,10 @@ func retake(now int64) uint32 {
 		s := pp.status
 		sysretake := false
 		if s == _Prunning || s == _Psyscall {
-			// Preempt G if it's running for too long.
+			// 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.
 			t := int64(pp.schedtick)
 			if int64(pd.schedtick) != t {
 				pd.schedtick = uint32(t)
@@ -6624,6 +6632,17 @@ 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
 	}