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SubjectRe: [PATCH] sched_clock: prevent scd->clock from moving backwards
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On Thu, 2008-10-09 at 17:17 +0200, Ingo Molnar wrote:

> hm, -tip testing found a sporadic hard lockup during bootup, and i've
> bisected it back to this patch. They happened on 64-bit test-systems.
> I've attached the .config that produced the problem.
>
> i reverted the patch and the lockups went away. But i cannot see what's
> wrong with it ...

I could have sworn I ran with the patch, but maybe I got my patch queue
messed up and never tested it right.

I think I see the problem.

--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -118,13 +118,13 @@ static u64 __update_sched_clock(struct
sched_clock_data *scd, u64 now)

/*
* scd->clock = clamp(scd->tick_gtod + delta,
- * max(scd->tick_gtod, scd->clock),
- * scd->tick_gtod + TICK_NSEC);
+ * max(scd->tick_gtod, scd->clock),
+ * min(scd->clock, scd->tick_gtod +
TICK_NSEC));
*/

clock = scd->tick_gtod + delta;
min_clock = wrap_max(scd->tick_gtod, scd->clock);
- max_clock = scd->tick_gtod + TICK_NSEC;
+ max_clock = wrap_min(scd->clock, scd->tick_gtod + TICK_NSEC);

clock = wrap_max(clock, min_clock);
clock = wrap_min(clock, max_clock);
We want wrap_max(scd->clock, scd->tick_gtod + TICK_NSEC), not
wrap_min(). The problem I am trying to fix is that scd->tick_gtod +
TICK_NSEC may be too low. The upper bound needs to be at LEAST
scd->clock. Limiting it to scd->clock all the time is disastrous. :-)

I'll fix the patch and retest it before sending it again.

Sorry about my sloppiness.

Shaggy
--
David Kleikamp
IBM Linux Technology Center



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