runtime: converge duplicate calls to netpollBreak into one

There might be some concurrent (maybe not concurrent, just sequential but in a short time window) and duplicate calls to `netpollBreak`, trying to wake up a net-poller. If one has called `netpollBreak` and that waking event hasn't been received by epollwait/kevent/..., then the subsequent calls of `netpollBreak` ought to be ignored or in other words, these calls should be converged into one.

Benchmarks go1.13.5 darwin/amd64:

benchmark-func           time/op (old)  time/op (new)  delta
BenchmarkNetpollBreak-4  29668ns ±1%    3131ns ±2%     -89.45%

mem/B (old)  mem/B (new)  delta
154B ±13%    0B ±0%       -100%

Change-Id: I3cf757a5d6edc5a99adad7aea3baee4b7f2a8f5c
GitHub-Last-Rev: 15bcfbab8a5db51f65da01315a5880a5dbf9e028
GitHub-Pull-Request: golang/go#36294
Reviewed-on: https://go-review.googlesource.com/c/go/+/212737
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 19 insertions, 11 deletions

diff --git a/src/runtime/netpoll_kqueue.go b/src/runtime/netpoll_kqueue.go
index 39d402252d..2ff21d8fcb 100644
--- a/src/runtime/netpoll_kqueue.go
+++ b/src/runtime/netpoll_kqueue.go
@@ -8,12 +8,17 @@ package runtime
 
 // Integrated network poller (kqueue-based implementation).
 
-import "unsafe"
+import (
+	"runtime/internal/atomic"
+	"unsafe"
+)
 
 var (
 	kq int32 = -1
 
 	netpollBreakRd, netpollBreakWr uintptr // for netpollBreak
+
+	netpollWakeSig uintptr // used to avoid duplicate calls of netpollBreak
 )
 
 func netpollinit() {
@@ -78,17 +83,19 @@ func netpollarm(pd *pollDesc, mode int) {
 
 // netpollBreak interrupts a kevent.
 func netpollBreak() {
-	for {
-		var b byte
-		n := write(netpollBreakWr, unsafe.Pointer(&b), 1)
-		if n == 1 || n == -_EAGAIN {
-			break
-		}
-		if n == -_EINTR {
-			continue
+	if atomic.Casuintptr(&netpollWakeSig, 0, 1) {
+		for {
+			var b byte
+			n := write(netpollBreakWr, unsafe.Pointer(&b), 1)
+			if n == 1 || n == -_EAGAIN {
+				break
+			}
+			if n == -_EINTR {
+				continue
+			}
+			println("runtime: netpollBreak write failed with", -n)
+			throw("runtime: netpollBreak write failed")
 		}
-		println("runtime: netpollBreak write failed with", -n)
-		throw("runtime: netpollBreak write failed")
 	}
 }
 
@@ -145,6 +152,7 @@ retry:
 				// if blocking.
 				var tmp [16]byte
 				read(int32(netpollBreakRd), noescape(unsafe.Pointer(&tmp[0])), int32(len(tmp)))
+				atomic.Storeuintptr(&netpollWakeSig, 0)
 			}
 			continue
 		}