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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

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>

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;
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);

return result;
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