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    Subjectgetcpu after sched_setaffinity
    The attached test program fails on a dual core (and probably SMP) 
    machine on x86-64. Depending on where the thread starts, in one of the
    iterations the sched_setffinity() call succeeds but then sched_getcpu()
    fails to report the correct CPU.

    In set_cpus_allowed migrate_task() is called if the new CPU set does not
    include the current CPU. I hope that migrate_task() also works for
    p==current.

    This leaves the x86-64 vgetcpu() implementation as the weak point. Is
    the caching causing problems? Should migrate_task() make sure the cache
    is reset?

    You need a very recent glibc to compile (glibc-2.5.90-22 in rawhide).
    If this is not available replace the sched_getcpu() call. But make sure
    you pass a pointer to a cache.

    --
    ➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
    #include <errno.h>
    #include <stdio.h>
    #include <sched.h>


    int
    main (void)
    {
    cpu_set_t cs;
    if (sched_getaffinity (getpid (), sizeof (cs), &cs) != 0)
    {
    printf ("getaffinity failed: %m\n");
    return 1;
    }

    int result = 0;
    int cpu = 0;
    while (CPU_COUNT (&cs) != 0)
    {
    if (CPU_ISSET (cpu, &cs))
    {
    cpu_set_t cs2;
    CPU_ZERO (&cs2);
    CPU_SET (cpu, &cs2);
    if (sched_setaffinity (getpid (), sizeof (cs2), &cs2) != 0)
    {
    printf ("setaffinity(%d) failed: %m\n", cpu);
    result = 1;
    }
    else
    {
    int cpu2 = sched_getcpu ();
    if (cpu2 == -1 && errno == ENOSYS)
    {
    puts ("getcpu syscall not implemented");
    return 0;
    }
    if (cpu2 != cpu)
    {
    printf ("getcpu results %d not possible\n", cpu2);
    result = 1;
    }
    }
    CPU_CLR (cpu, &cs);
    }
    ++cpu;
    }

    return result;
    }
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