1// Copyright 2019 The Go Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style 3// license that can be found in the LICENSE file. 4 5package runtime_test 6 7import ( 8 . "runtime" 9 "sync/atomic" 10 "testing" 11) 12 13// TestSemaHandoff checks that when semrelease+handoff is 14// requested, the G that releases the semaphore yields its 15// P directly to the first waiter in line. 16// See issue 33747 for discussion. 17func TestSemaHandoff(t *testing.T) { 18 const iter = 10000 19 ok := 0 20 for i := 0; i < iter; i++ { 21 if testSemaHandoff() { 22 ok++ 23 } 24 } 25 // As long as two thirds of handoffs are direct, we 26 // consider the test successful. The scheduler is 27 // nondeterministic, so this test checks that we get the 28 // desired outcome in a significant majority of cases. 29 // The actual ratio of direct handoffs is much higher 30 // (>90%) but we use a lower threshold to minimize the 31 // chances that unrelated changes in the runtime will 32 // cause the test to fail or become flaky. 33 if ok < iter*2/3 { 34 t.Fatal("direct handoff < 2/3:", ok, iter) 35 } 36} 37 38func TestSemaHandoff1(t *testing.T) { 39 if GOMAXPROCS(-1) <= 1 { 40 t.Skip("GOMAXPROCS <= 1") 41 } 42 defer GOMAXPROCS(GOMAXPROCS(-1)) 43 GOMAXPROCS(1) 44 TestSemaHandoff(t) 45} 46 47func TestSemaHandoff2(t *testing.T) { 48 if GOMAXPROCS(-1) <= 2 { 49 t.Skip("GOMAXPROCS <= 2") 50 } 51 defer GOMAXPROCS(GOMAXPROCS(-1)) 52 GOMAXPROCS(2) 53 TestSemaHandoff(t) 54} 55 56func testSemaHandoff() bool { 57 var sema, res uint32 58 done := make(chan struct{}) 59 60 // We're testing that the current goroutine is able to yield its time slice 61 // to another goroutine. Stop the current goroutine from migrating to 62 // another CPU where it can win the race (and appear to have not yielded) by 63 // keeping the CPUs slightly busy. 64 for i := 0; i < GOMAXPROCS(-1); i++ { 65 go func() { 66 for { 67 select { 68 case <-done: 69 return 70 default: 71 } 72 Gosched() 73 } 74 }() 75 } 76 77 go func() { 78 Semacquire(&sema) 79 atomic.CompareAndSwapUint32(&res, 0, 1) 80 81 Semrelease1(&sema, true, 0) 82 close(done) 83 }() 84 for SemNwait(&sema) == 0 { 85 Gosched() // wait for goroutine to block in Semacquire 86 } 87 88 // The crux of the test: we release the semaphore with handoff 89 // and immediately perform a CAS both here and in the waiter; we 90 // want the CAS in the waiter to execute first. 91 Semrelease1(&sema, true, 0) 92 atomic.CompareAndSwapUint32(&res, 0, 2) 93 94 <-done // wait for goroutines to finish to avoid data races 95 96 return res == 1 // did the waiter run first? 97} 98