sema_test.go

Documentation: runtime

     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  
     5  package runtime_test
     6  
     7  import (
     8  	. "runtime"
     9  	"sync"
    10  	"sync/atomic"
    11  	"testing"
    12  )
    13  
    14  // TestSemaHandoff checks that when semrelease+handoff is
    15  // requested, the G that releases the semaphore yields its
    16  // P directly to the first waiter in line.
    17  // See issue 33747 for discussion.
    18  func TestSemaHandoff(t *testing.T) {
    19  	const iter = 10000
    20  	ok := 0
    21  	for i := 0; i < iter; i++ {
    22  		if testSemaHandoff() {
    23  			ok++
    24  		}
    25  	}
    26  	// As long as two thirds of handoffs are direct, we
    27  	// consider the test successful. The scheduler is
    28  	// nondeterministic, so this test checks that we get the
    29  	// desired outcome in a significant majority of cases.
    30  	// The actual ratio of direct handoffs is much higher
    31  	// (>90%) but we use a lower threshold to minimize the
    32  	// chances that unrelated changes in the runtime will
    33  	// cause the test to fail or become flaky.
    34  	if ok < iter*2/3 {
    35  		t.Fatal("direct handoff < 2/3:", ok, iter)
    36  	}
    37  }
    38  
    39  func TestSemaHandoff1(t *testing.T) {
    40  	if GOMAXPROCS(-1) <= 1 {
    41  		t.Skip("GOMAXPROCS <= 1")
    42  	}
    43  	defer GOMAXPROCS(GOMAXPROCS(-1))
    44  	GOMAXPROCS(1)
    45  	TestSemaHandoff(t)
    46  }
    47  
    48  func TestSemaHandoff2(t *testing.T) {
    49  	if GOMAXPROCS(-1) <= 2 {
    50  		t.Skip("GOMAXPROCS <= 2")
    51  	}
    52  	defer GOMAXPROCS(GOMAXPROCS(-1))
    53  	GOMAXPROCS(2)
    54  	TestSemaHandoff(t)
    55  }
    56  
    57  func testSemaHandoff() bool {
    58  	var sema, res uint32
    59  	done := make(chan struct{})
    60  
    61  	// We're testing that the current goroutine is able to yield its time slice
    62  	// to another goroutine. Stop the current goroutine from migrating to
    63  	// another CPU where it can win the race (and appear to have not yielded) by
    64  	// keeping the CPUs slightly busy.
    65  	var wg sync.WaitGroup
    66  	for i := 0; i < GOMAXPROCS(-1); i++ {
    67  		wg.Add(1)
    68  		go func() {
    69  			defer wg.Done()
    70  			for {
    71  				select {
    72  				case <-done:
    73  					return
    74  				default:
    75  				}
    76  				Gosched()
    77  			}
    78  		}()
    79  	}
    80  
    81  	wg.Add(1)
    82  	go func() {
    83  		defer wg.Done()
    84  		Semacquire(&sema)
    85  		atomic.CompareAndSwapUint32(&res, 0, 1)
    86  
    87  		Semrelease1(&sema, true, 0)
    88  		close(done)
    89  	}()
    90  	for SemNwait(&sema) == 0 {
    91  		Gosched() // wait for goroutine to block in Semacquire
    92  	}
    93  
    94  	// The crux of the test: we release the semaphore with handoff
    95  	// and immediately perform a CAS both here and in the waiter; we
    96  	// want the CAS in the waiter to execute first.
    97  	Semrelease1(&sema, true, 0)
    98  	atomic.CompareAndSwapUint32(&res, 0, 2)
    99  
   100  	wg.Wait() // wait for goroutines to finish to avoid data races
   101  
   102  	return res == 1 // did the waiter run first?
   103  }
   104
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