From b4fe51823d797d6959b2eee7868023e61606daa9 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Wed, 23 Sep 2009 15:56:47 -0700
Subject: 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>
---
 kernel/exit.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/exit.c b/kernel/exit.c
index 7838b4d..270a68b 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);
 }
 
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