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SubjectRe: [PATCH] sched_clock: prevent scd->clock from moving backwards
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On Wed, 2008-10-08 at 08:00 -0500, Dave Kleikamp wrote:
> sched_clock: prevent scd->clock from moving backwards
>
> When sched_clock_cpu() couples the clocks between two cpus, it may
> increment scd->clock beyond the GTOD tick window that __update_sched_clock()
> uses to clamp the clock. A later call to __update_sched_clock() may move
> the clock back to scd->tick_gtod + TICK_NSEC, violating the clock's
> monotonic property.
>
> This patch ensures that scd->clock will not be set backward.

Ah, yes indeed, this comes from the tick not happening at the same time
on different cpus, so if we use a local timestamp to move a remote clock
forward, this scenario could indeed happen.

The fix looks good to me, good catch, thanks shaggy!

A related 'fix' which I'm still not quite sure about is making the
window 'tick_gtod + 2*TICK_NSEC'. That increases the max observed
difference to 4 jiffies, but allows ticks to be 'late' a bit without
first holding back time and then jumping ahead again.

> Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

>
> diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
> index e8ab096..a989d64 100644
> --- 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);
>
>



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