Commit 152f9d07 authored Sep 30, 2009 by Eric Dumazet Committed by Ingo Molnar Sep 30, 2009

sched_clock: Fix atomicity/continuity bug by using cmpxchg64()



Commit def0a9b2 (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@de.ibm.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090930170754.0886ff2e@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 79e1dd05

kernel/sched_clock.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -127,7 +127,7 @@ static u64 sched_clock_local(struct sched_clock_data *scd)
		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 @@ static u64 sched_clock_remote(struct sched_clock_data *scd)
		val = remote_clock;
		}

		if (cmpxchg(ptr, old_val, val) != old_val)
		if (cmpxchg64(ptr, old_val, val) != old_val)
		goto again;

		return val;