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Revert "sync: yield to the waiter when unlocking a starving mutex"

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This reverts CL 200577.

Reason for revert: broke linux-arm64-packet and solaris-amd64-oraclerel builders

Fixes #35424
Updates #33747

Change-Id: I2575fd84d37995d458183caae54704f15d8b8426
Reviewed-on: https://go-review.googlesource.com/c/go/+/205817
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This commit is contained in:
Bryan C. Mills 2019-11-07 14:09:23 +00:00
parent e8f01d591f
commit 73d57bf80f
5 changed files with 2 additions and 125 deletions
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8
src/runtime/export_test.go
View File

@ -730,11 +730,3 @@ func RunGetgThreadSwitchTest() {
panic("g1 != g3")
}
}
var Semacquire = semacquire
var Semrelease1 = semrelease1
func SemNwait(addr *uint32) uint32 {
root := semroot(addr)
return atomic.Load(&root.nwait)
}

15
src/runtime/proc.go
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@ -2753,22 +2753,7 @@ func preemptPark(gp *g) {
casGToPreemptScan(gp, _Grunning, _Gscan|_Gpreempted)
dropg()
casfrom_Gscanstatus(gp, _Gscan|_Gpreempted, _Gpreempted)
schedule()
}
// goyield is like Gosched, but it:
// - does not emit a GoSched trace event
// - puts the current G on the runq of the current P instead of the globrunq
func goyield() {
checkTimeouts()
mcall(goyield_m)
}
func goyield_m(gp *g) {
pp := gp.m.p.ptr()
casgstatus(gp, _Grunning, _Grunnable)
dropg()
runqput(pp, gp, false)
schedule()
}

21
src/runtime/sema.go
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@ -180,7 +180,7 @@ func semrelease1(addr *uint32, handoff bool, skipframes int) {
atomic.Xadd(&root.nwait, -1)
}
unlock(&root.lock)
if s != nil { // May be slow or even yield, so unlock first
if s != nil { // May be slow, so unlock first
acquiretime := s.acquiretime
if acquiretime != 0 {
mutexevent(t0-acquiretime, 3+skipframes)
@ -192,25 +192,6 @@ func semrelease1(addr *uint32, handoff bool, skipframes int) {
s.ticket = 1
}
readyWithTime(s, 5+skipframes)
if s.ticket == 1 {
// Direct G handoff
// readyWithTime has added the waiter G as runnext in the
// current P; we now call the scheduler so that we start running
// the waiter G immediately.
// Note that waiter inherits our time slice: this is desirable
// to avoid having a highly contended semaphore hog the P
// indefinitely. goyield is like Gosched, but it does not emit a
// GoSched trace event and, more importantly, puts the current G
// on the local runq instead of the global one.
// We only do this in the starving regime (handoff=true), as in
// the non-starving case it is possible for a different waiter
// to acquire the semaphore while we are yielding/scheduling,
// and this would be wasteful. We wait instead to enter starving
// regime, and then we start to do direct handoffs of ticket and
// P.
// See issue 33747 for discussion.
goyield()
}
}
}

80
src/runtime/sema_test.go
View File

@ -1,80 +0,0 @@
// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package runtime_test
import (
. "runtime"
"sync/atomic"
"testing"
)
// TestSemaHandoff checks that when semrelease+handoff is
// requested, the G that releases the semaphore yields its
// P directly to the first waiter in line.
// See issue 33747 for discussion.
func TestSemaHandoff(t *testing.T) {
const iter = 10000
ok := 0
for i := 0; i < iter; i++ {
if testSemaHandoff() {
ok++
}
}
// As long as two thirds of handoffs are direct, we
// consider the test successful. The scheduler is
// nondeterministic, so this test checks that we get the
// desired outcome in a significant majority of cases.
// The actual ratio of direct handoffs is much higher
// (>90%) but we use a lower threshold to minimize the
// chances that unrelated changes in the runtime will
// cause the test to fail or become flaky.
if ok < iter*2/3 {
t.Fatal("direct handoff < 2/3:", ok, iter)
}
}
func TestSemaHandoff1(t *testing.T) {
if GOMAXPROCS(-1) <= 1 {
t.Skip("GOMAXPROCS <= 1")
}
defer GOMAXPROCS(GOMAXPROCS(-1))
GOMAXPROCS(1)
TestSemaHandoff(t)
}
func TestSemaHandoff2(t *testing.T) {
if GOMAXPROCS(-1) <= 2 {
t.Skip("GOMAXPROCS <= 2")
}
defer GOMAXPROCS(GOMAXPROCS(-1))
GOMAXPROCS(2)
TestSemaHandoff(t)
}
func testSemaHandoff() bool {
var sema, res uint32
done := make(chan struct{})
go func() {
Semacquire(&sema)
atomic.CompareAndSwapUint32(&res, 0, 1)
Semrelease1(&sema, true, 0)
close(done)
}()
for SemNwait(&sema) == 0 {
Gosched() // wait for goroutine to block in Semacquire
}
// The crux of the test: we release the semaphore with handoff
// and immediately perform a CAS both here and in the waiter; we
// want the CAS in the waiter to execute first.
Semrelease1(&sema, true, 0)
atomic.CompareAndSwapUint32(&res, 0, 2)
<-done // wait for goroutines to finish to avoid data races
return res == 1 // did the waiter run first?
}

3
src/sync/mutex.go
View File

@ -216,8 +216,7 @@ func (m *Mutex) unlockSlow(new int32) {
old = m.state
}
} else {
// Starving mode: handoff mutex ownership to the next waiter, and yield
// our time slice so that the next waiter can start to run immediately.
// Starving mode: handoff mutex ownership to the next waiter.
// Note: mutexLocked is not set, the waiter will set it after wakeup.
// But mutex is still considered locked if mutexStarving is set,
// so new coming goroutines won't acquire it.