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>

1 files changed, 3 insertions, 0 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 7838b4d68774..270a68b7f22f 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1581,6 +1581,9 @@ static int child_wait_callback(wait_queue_t *wait, unsigned mode,
         if (!eligible_child(wo, p))
                 return 0;
 
+        if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent)
+                return 0;
+
         return default_wake_function(wait, mode, sync, key);
 }