From 4166ff42c09cae4ca9e15154627e7cfc80586c65 Mon Sep 17 00:00:00 2001
From: Andrei Vagin <avagin@google.com>
Date: Fri, 29 Mar 2019 10:43:31 -0700
Subject: [PATCH] runtime: preempt a goroutine which calls a lot of short
 system calls

A goroutine should be preempted if it runs for 10ms without blocking.
We found that this doesn't work for goroutines which call short system calls.

For example, the next program can stuck for seconds without this fix:

$ cat main.go
package main

import (
	"runtime"
	"syscall"
)

func main() {
	runtime.GOMAXPROCS(1)
	c := make(chan int)
	go func() {
		c <- 1
		for {
			t := syscall.Timespec{
				Nsec: 300,
			}
			if true {
				syscall.Nanosleep(&t, nil)
			}
		}
	}()
	<-c
}

$ time go run main.go

real	0m8.796s
user	0m0.367s
sys	0m0.893s

Updates #10958

Change-Id: Id3be54d3779cc28bfc8b33fe578f13778f1ae2a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/170138
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
---
 src/runtime/export_test.go |  2 ++
 src/runtime/proc.go        | 28 ++++++++--------
 src/runtime/proc_test.go   | 67 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 84 insertions(+), 13 deletions(-)

diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 9eaf92dc7c..a16e664895 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -34,6 +34,8 @@ var Fastlog2 = fastlog2
 var Atoi = atoi
 var Atoi32 = atoi32
 
+var Nanotime = nanotime
+
 type LFNode struct {
 	Next    uint64
 	Pushcnt uintptr
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 6b5b3e2b2b..29763d328a 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -4380,10 +4380,24 @@ func retake(now int64) uint32 {
 		}
 		pd := &_p_.sysmontick
 		s := _p_.status
+		sysretake := false
+		if s == _Prunning || s == _Psyscall {
+			// Preempt G if it's running for too long.
+			t := int64(_p_.schedtick)
+			if int64(pd.schedtick) != t {
+				pd.schedtick = uint32(t)
+				pd.schedwhen = now
+			} else if pd.schedwhen+forcePreemptNS <= now {
+				preemptone(_p_)
+				// In case of syscall, preemptone() doesn't
+				// work, because there is no M wired to P.
+				sysretake = true
+			}
+		}
 		if s == _Psyscall {
 			// Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
 			t := int64(_p_.syscalltick)
-			if int64(pd.syscalltick) != t {
+			if !sysretake && int64(pd.syscalltick) != t {
 				pd.syscalltick = uint32(t)
 				pd.syscallwhen = now
 				continue
@@ -4412,18 +4426,6 @@ func retake(now int64) uint32 {
 			}
 			incidlelocked(1)
 			lock(&allpLock)
-		} else if s == _Prunning {
-			// Preempt G if it's running for too long.
-			t := int64(_p_.schedtick)
-			if int64(pd.schedtick) != t {
-				pd.schedtick = uint32(t)
-				pd.schedwhen = now
-				continue
-			}
-			if pd.schedwhen+forcePreemptNS > now {
-				continue
-			}
-			preemptone(_p_)
 		}
 	}
 	unlock(&allpLock)
diff --git a/src/runtime/proc_test.go b/src/runtime/proc_test.go
index 1715324aa0..09b0652bee 100644
--- a/src/runtime/proc_test.go
+++ b/src/runtime/proc_test.go
@@ -5,6 +5,7 @@
 package runtime_test
 
 import (
+	"fmt"
 	"math"
 	"net"
 	"runtime"
@@ -910,3 +911,69 @@ func TestLockOSThreadAvoidsStatePropagation(t *testing.T) {
 		t.Errorf("want %q, got %q", want, output)
 	}
 }
+
+// fakeSyscall emulates a system call.
+//go:nosplit
+func fakeSyscall(duration time.Duration) {
+	runtime.Entersyscall()
+	for start := runtime.Nanotime(); runtime.Nanotime()-start < int64(duration); {
+	}
+	runtime.Exitsyscall()
+}
+
+// Check that a goroutine will be preempted if it is calling short system calls.
+func testPreemptionAfterSyscall(t *testing.T, syscallDuration time.Duration) {
+	if runtime.GOARCH == "wasm" {
+		t.Skip("no preemption on wasm yet")
+	}
+
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(2))
+
+	interations := 10
+	if testing.Short() {
+		interations = 1
+	}
+	const (
+		maxDuration = 3 * time.Second
+		nroutines   = 8
+	)
+
+	for i := 0; i < interations; i++ {
+		c := make(chan bool, nroutines)
+		stop := uint32(0)
+
+		start := time.Now()
+		for g := 0; g < nroutines; g++ {
+			go func(stop *uint32) {
+				c <- true
+				for atomic.LoadUint32(stop) == 0 {
+					fakeSyscall(syscallDuration)
+				}
+				c <- true
+			}(&stop)
+		}
+		// wait until all goroutines have started.
+		for g := 0; g < nroutines; g++ {
+			<-c
+		}
+		atomic.StoreUint32(&stop, 1)
+		// wait until all goroutines have finished.
+		for g := 0; g < nroutines; g++ {
+			<-c
+		}
+		duration := time.Since(start)
+
+		if duration > maxDuration {
+			t.Errorf("timeout exceeded: %v (%v)", duration, maxDuration)
+		}
+	}
+}
+
+func TestPreemptionAfterSyscall(t *testing.T) {
+	for _, i := range []time.Duration{10, 100, 1000} {
+		d := i * time.Microsecond
+		t.Run(fmt.Sprint(d), func(t *testing.T) {
+			testPreemptionAfterSyscall(t, d)
+		})
+	}
+}
-- 
2.50.0