[tip:sched/urgent] sched_clock: Fix atomicity/continuity bug by using cmpxchg64()
From: tip-bot for Eric Dumazet
Date: Wed Sep 30 2009 - 16:41:39 EST
Commit-ID: 623c4f89c47cca0088014388211d8bc0a5358f0a
Gitweb: http://git.kernel.org/tip/623c4f89c47cca0088014388211d8bc0a5358f0a
Author: Eric Dumazet <eric.dumazet@xxxxxxxxx>
AuthorDate: Wed, 30 Sep 2009 20:36:19 +0200
Committer: Ingo Molnar <mingo@xxxxxxx>
CommitDate: Wed, 30 Sep 2009 22:36:12 +0200
sched_clock: Fix atomicity/continuity bug by using cmpxchg64()
Commit def0a9b2573 (sched_clock: Make it NMI safe) assumed
cmpxchg() of 64bit values was available on X86_32.
That is not so - and causes some subtle scheduler misbehavior due
to incorrect timestamps off to up by ~4 seconds.
Two symptoms are known right now:
- interactivity problems seen by Arjan: up to 600 msecs
latencies instead of the expected 20-40 msecs. These
latencies are very visible on the desktop.
- incorrect CPU stats: occasionally too high percentages in 'top',
and crazy CPU usage stats.
Reported-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: John Stultz <johnstul@xxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
LKML-Reference: <20090930170754.0886ff2e@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
kernel/sched_clock.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index ac2e1dc..479ce56 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -127,7 +127,7 @@ again:
clock = wrap_max(clock, min_clock);
clock = wrap_min(clock, max_clock);
- if (cmpxchg(&scd->clock, old_clock, clock) != old_clock)
+ if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
goto again;
return clock;
@@ -163,7 +163,7 @@ again:
val = remote_clock;
}
- if (cmpxchg(ptr, old_val, val) != old_val)
+ if (cmpxchg64(ptr, old_val, val) != old_val)
goto again;
return val;
--
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