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: 15bcfbab8a
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>

src/runtime/netpoll_epoll.go

@@ -6,7 +6,10 @@
 
 package runtime
 
-import "unsafe"
+import (
+	"runtime/internal/atomic"
+	"unsafe"
+)
 
 func epollcreate(size int32) int32
 func epollcreate1(flags int32) int32
@@ -22,6 +25,8 @@ var (
 	epfd int32 = -1 // epoll descriptor
 
 	netpollBreakRd, netpollBreakWr uintptr // for netpollBreak
+
+	netpollWakeSig uintptr // used to avoid duplicate calls of netpollBreak
 )
 
 func netpollinit() {
@@ -74,20 +79,22 @@ func netpollarm(pd *pollDesc, mode int) {
 
 // netpollBreak interrupts an epollwait.
 func netpollBreak() {
-	for {
-		var b byte
-		n := write(netpollBreakWr, unsafe.Pointer(&b), 1)
-		if n == 1 {
-			break
+	if atomic.Casuintptr(&netpollWakeSig, 0, 1) {
+		for {
+			var b byte
+			n := write(netpollBreakWr, unsafe.Pointer(&b), 1)
+			if n == 1 {
+				break
+			}
+			if n == -_EINTR {
+				continue
+			}
+			if n == -_EAGAIN {
+				return
+			}
+			println("runtime: netpollBreak write failed with", -n)
+			throw("runtime: netpollBreak write failed")
 		}
-		if n == -_EINTR {
-			continue
-		}
-		if n == -_EAGAIN {
-			return
-		}
-		println("runtime: netpollBreak write failed with", -n)
-		throw("runtime: netpollBreak write failed")
 	}
 }
 
@@ -147,6 +154,7 @@ retry:
 				// if blocking.
 				var tmp [16]byte
 				read(int32(netpollBreakRd), noescape(unsafe.Pointer(&tmp[0])), int32(len(tmp)))
+				atomic.Storeuintptr(&netpollWakeSig, 0)
 			}
 			continue
 		}

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 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")
 		}
-		if n == -_EINTR {
-			continue
-		}
-		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
 		}

src/runtime/netpoll_os_test.go

@@ -0,0 +1,28 @@
+package runtime_test
+
+import (
+	"runtime"
+	"sync"
+	"testing"
+)
+
+var wg sync.WaitGroup
+
+func init() {
+	runtime.NetpollGenericInit()
+}
+
+func BenchmarkNetpollBreak(b *testing.B) {
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < 10; j++ {
+			wg.Add(1)
+			go func() {
+				runtime.NetpollBreak()
+				wg.Done()
+			}()
+		}
+	}
+	wg.Wait()
+	b.StopTimer()
+}

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
 		}

src/runtime/netpoll_windows.go

@@ -5,6 +5,7 @@
 package runtime
 
 import (
+	"runtime/internal/atomic"
 	"unsafe"
 )
 
@@ -31,7 +32,11 @@ type overlappedEntry struct {
 	qty      uint32
 }
 
-var iocphandle uintptr = _INVALID_HANDLE_VALUE // completion port io handle
+var (
+	iocphandle uintptr = _INVALID_HANDLE_VALUE // completion port io handle
+
+	netpollWakeSig uintptr // used to avoid duplicate calls of netpollBreak
+)
 
 func netpollinit() {
 	iocphandle = stdcall4(_CreateIoCompletionPort, _INVALID_HANDLE_VALUE, 0, 0, _DWORD_MAX)
@@ -62,9 +67,11 @@ func netpollarm(pd *pollDesc, mode int) {
 }
 
 func netpollBreak() {
-	if stdcall4(_PostQueuedCompletionStatus, iocphandle, 0, 0, 0) == 0 {
-		println("runtime: netpoll: PostQueuedCompletionStatus failed (errno=", getlasterror(), ")")
-		throw("runtime: netpoll: PostQueuedCompletionStatus failed")
+	if atomic.Casuintptr(&netpollWakeSig, 0, 1) {
+		if stdcall4(_PostQueuedCompletionStatus, iocphandle, 0, 0, 0) == 0 {
+			println("runtime: netpoll: PostQueuedCompletionStatus failed (errno=", getlasterror(), ")")
+			throw("runtime: netpoll: PostQueuedCompletionStatus failed")
+		}
 	}
 }
 
@@ -126,6 +133,7 @@ func netpoll(delay int64) gList {
 			}
 			handlecompletion(&toRun, op, errno, qty)
 		} else {
+			atomic.Storeuintptr(&netpollWakeSig, 0)
 			if delay == 0 {
 				// Forward the notification to the
 				// blocked poller.