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, 17 insertions, 10 deletions

diff --git a/src/runtime/netpoll_solaris.go b/src/runtime/netpoll_solaris.go
index 15818cb4ea..34b3ee9308 100644
--- a/src/runtime/netpoll_solaris.go
+++ b/src/runtime/netpoll_solaris.go
@@ -4,7 +4,10 @@
 
 package runtime
 
-import "unsafe"
+import (
+	"runtime/internal/atomic"
+	"unsafe"
+)
 
 // Solaris runtime-integrated network poller.
 //
@@ -85,6 +88,7 @@ var (
 	libc_port_dissociate,
 	libc_port_getn,
 	libc_port_alert libcFunc
+	netpollWakeSig uintptr // used to avoid duplicate calls of netpollBreak
 )
 
 func errno() int32 {
@@ -187,15 +191,17 @@ func netpollarm(pd *pollDesc, mode int) {
 
 // netpollBreak interrupts a port_getn wait.
 func netpollBreak() {
-	// Use port_alert to put portfd into alert mode.
-	// This will wake up all threads sleeping in port_getn on portfd,
-	// and cause their calls to port_getn to return immediately.
-	// Further, until portfd is taken out of alert mode,
-	// all calls to port_getn will return immediately.
-	if port_alert(portfd, _PORT_ALERT_UPDATE, _POLLHUP, uintptr(unsafe.Pointer(&portfd))) < 0 {
-		if e := errno(); e != _EBUSY {
-			println("runtime: port_alert failed with", e)
-			throw("runtime: netpoll: port_alert failed")
+	if atomic.Casuintptr(&netpollWakeSig, 0, 1) {
+		// Use port_alert to put portfd into alert mode.
+		// This will wake up all threads sleeping in port_getn on portfd,
+		// and cause their calls to port_getn to return immediately.
+		// Further, until portfd is taken out of alert mode,
+		// all calls to port_getn will return immediately.
+		if port_alert(portfd, _PORT_ALERT_UPDATE, _POLLHUP, uintptr(unsafe.Pointer(&portfd))) < 0 {
+			if e := errno(); e != _EBUSY {
+				println("runtime: port_alert failed with", e)
+				throw("runtime: netpoll: port_alert failed")
+			}
 		}
 	}
 }
@@ -268,6 +274,7 @@ retry:
 					println("runtime: port_alert failed with", e)
 					throw("runtime: netpoll: port_alert failed")
 				}
+				atomic.Storeuintptr(&netpollWakeSig, 0)
 			}
 			continue
 		}