do_wait() wakeup optimization: child_wait_callback: check __WNOTHREAD case
Suggested by Roland. do_wait(__WNOTHREAD) can only succeed if the caller is either ptracer, or it is ->real_parent and the child is not traced. IOW, caller == p->parent otherwise we should not wake up. Change child_wait_callback() to check this. Ratan reports the workload with CPU load >99% caused by unnecessary wakeups, should be fixed by this patch. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Acked-by: Roland McGrath <roland@redhat.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Ratan Nalumasu <rnalumasu@gmail.com> Cc: Vitaly Mayatskikh <vmayatsk@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -1581,6 +1581,9 @@ static int child_wait_callback(wait_queue_t *wait, unsigned mode,
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if (!eligible_child(wo, p))
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return 0;
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if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent)
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return 0;
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return default_wake_function(wait, mode, sync, key);
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}
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