103 lines
2.5 KiB
Go
103 lines
2.5 KiB
Go
// Copyright 2019 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package runtime_test
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import (
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. "runtime"
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"sync"
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"sync/atomic"
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"testing"
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)
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// TestSemaHandoff checks that when semrelease+handoff is
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// requested, the G that releases the semaphore yields its
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// P directly to the first waiter in line.
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// See issue 33747 for discussion.
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func TestSemaHandoff(t *testing.T) {
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const iter = 10000
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ok := 0
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for i := 0; i < iter; i++ {
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if testSemaHandoff() {
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ok++
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}
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}
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// As long as two thirds of handoffs are direct, we
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// consider the test successful. The scheduler is
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// nondeterministic, so this test checks that we get the
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// desired outcome in a significant majority of cases.
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// The actual ratio of direct handoffs is much higher
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// (>90%) but we use a lower threshold to minimize the
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// chances that unrelated changes in the runtime will
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// cause the test to fail or become flaky.
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if ok < iter*2/3 {
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t.Fatal("direct handoff < 2/3:", ok, iter)
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}
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}
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func TestSemaHandoff1(t *testing.T) {
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if GOMAXPROCS(-1) <= 1 {
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t.Skip("GOMAXPROCS <= 1")
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}
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defer GOMAXPROCS(GOMAXPROCS(-1))
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GOMAXPROCS(1)
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TestSemaHandoff(t)
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}
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func TestSemaHandoff2(t *testing.T) {
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if GOMAXPROCS(-1) <= 2 {
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t.Skip("GOMAXPROCS <= 2")
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}
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defer GOMAXPROCS(GOMAXPROCS(-1))
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GOMAXPROCS(2)
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TestSemaHandoff(t)
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}
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func testSemaHandoff() bool {
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var sema, res uint32
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done := make(chan struct{})
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// We're testing that the current goroutine is able to yield its time slice
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// to another goroutine. Stop the current goroutine from migrating to
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// another CPU where it can win the race (and appear to have not yielded) by
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// keeping the CPUs slightly busy.
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var wg sync.WaitGroup
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for i := 0; i < GOMAXPROCS(-1); i++ {
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wg.Add(1)
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go func() {
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defer wg.Done()
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for {
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select {
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case <-done:
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return
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default:
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}
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Gosched()
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}
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}()
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}
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wg.Add(1)
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go func() {
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defer wg.Done()
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Semacquire(&sema)
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atomic.CompareAndSwapUint32(&res, 0, 1)
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Semrelease1(&sema, true, 0)
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close(done)
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}()
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for SemNwait(&sema) == 0 {
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Gosched() // wait for goroutine to block in Semacquire
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}
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// The crux of the test: we release the semaphore with handoff
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// and immediately perform a CAS both here and in the waiter; we
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// want the CAS in the waiter to execute first.
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Semrelease1(&sema, true, 0)
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atomic.CompareAndSwapUint32(&res, 0, 2)
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wg.Wait() // wait for goroutines to finish to avoid data races
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return res == 1 // did the waiter run first?
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}
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