• Russ Cox's avatar
    runtime/pprof: disable test on darwin · 071d212a
    Russ Cox authored
    Fixes #1641.
    
    Actually it side steps the real issue, which is that the
    setitimer(2) implementation on OS X is not useful for
    profiling of multi-threaded programs.  I filed the below
    using the Apple Bug Reporter.
    
    /*
    Filed as Apple Bug Report #9177434.
    
    This program creates a new pthread that loops, wasting cpu time.
    In the main pthread, it sleeps on a condition that will never come true.
    Before doing so it sets up an interval timer using ITIMER_PROF.
    The handler prints a message saying which thread it is running on.
    
    POSIX does not specify which thread should receive the signal, but
    in order to be useful in a user-mode self-profiler like pprof or gprof
       http://code.google.com/p/google-perftools
       http://www.delorie.com/gnu/docs/binutils/gprof_25.html
    it is important that the thread that receives the signal is the one
    whose execution caused the timer to expire.
    
    Linux and FreeBSD handle this by sending the signal to the process's
    queue but delivering it to the current thread if possible:
    
       http://lxr.linux.no/linux+v2.6.38/kernel/signal.c#L802
         807        /*
         808         * Now find a thread we can wake up to take the signal off the queue.
         809         *
         810         * If the main thread wants the signal, it gets first crack.
         811         * Probably the least surprising to the average bear.
         812         * /
    
       http://fxr.watson.org/fxr/source/kern/kern_sig.c?v=FREEBSD8;im=bigexcerpts#L1907
         1914         /*
         1915          * Check if current thread can handle the signal without
         1916          * switching context to another thread.
         1917          * /
    
    On those operating systems, this program prints:
    
        $ ./a.out
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        signal on cpu-chewing looper thread
        $
    
    The OS X kernel does not have any such preference.  Its get_signalthread
    does not prefer current_thread(), in contrast to the other two systems,
    so the signal gets delivered to the first thread in the list that is able to
    handle it, which ends up being the main thread in this experiment.
    http://fxr.watson.org/fxr/source/bsd/kern/kern_sig.c?v=xnu-1456.1.26;im=excerpts#L1666
    
        $ ./a.out
        signal on sleeping main thread
        signal on sleeping main thread
        signal on sleeping main thread
        signal on sleeping main thread
        signal on sleeping main thread
        signal on sleeping main thread
        signal on sleeping main thread
        signal on sleeping main thread
        signal on sleeping main thread
        signal on sleeping main thread
        $
    
    The fix is to make get_signalthread use the same heuristic as
    Linux and FreeBSD, namely to use current_thread() if possible
    before scanning the process thread list.
    
    */
    
    #include <sys/time.h>
    #include <sys/signal.h>
    #include <pthread.h>
    #include <unistd.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>
    
    static void handler(int);
    static void* looper(void*);
    
    static pthread_t pmain, ploop;
    
    int
    main(void)
    {
            struct itimerval it;
            struct sigaction sa;
            pthread_cond_t cond;
            pthread_mutex_t mu;
    
            memset(&sa, 0, sizeof sa);
            sa.sa_handler = handler;
            sa.sa_flags = SA_RESTART;
            memset(&sa.sa_mask, 0xff, sizeof sa.sa_mask);
            sigaction(SIGPROF, &sa, 0);
    
            pmain = pthread_self();
            pthread_create(&ploop, 0, looper, 0);
    
            memset(&it, 0, sizeof it);
            it.it_interval.tv_usec = 10000;
            it.it_value = it.it_interval;
            setitimer(ITIMER_PROF, &it, 0);
    
            pthread_mutex_init(&mu, 0);
            pthread_mutex_lock(&mu);
    
            pthread_cond_init(&cond, 0);
            for(;;)
                    pthread_cond_wait(&cond, &mu);
    
            return 0;
    }
    
    static void
    handler(int sig)
    {
            static int nsig;
            pthread_t p;
    
            p = pthread_self();
            if(p == pmain)
                    printf("signal on sleeping main thread\n");
            else if(p == ploop)
                    printf("signal on cpu-chewing looper thread\n");
            else
                    printf("signal on %p\n", (void*)p);
            if(++nsig >= 10)
                    exit(0);
    }
    
    static void*
    looper(void *v)
    {
            for(;;);
    }
    
    R=r
    CC=golang-dev
    https://golang.org/cl/4273113
    071d212a
pprof_test.go 1.71 KB