Messages in this thread Patch in this message | | | Subject | Re: [PATCH] sched_clock: prevent scd->clock from moving backwards | From | Peter Zijlstra <> | Date | Thu, 09 Oct 2008 01:05:04 +0200 |
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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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