Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems

Robin Holt
2009-01-06T20:19:50Z

On Tue, Jan 06, 2009 at 12:15:34PM -0800, Luck, Tony wrote:
> > Turn on CONFIG_HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN.
> >
> > SGI Altix has unsynchronized itc clocks. This results in rq->clock
> > occasionally being set to a time in the past by a remote cpu.
>
> SGI Altix is definitely the worst offender here. On heterogeneneous
> systems with different model cpus across nodes the itc rate may differ
> by hundreds of megahertz. Even when all cpus are at the same rated
> speed, different nodes are driven from different crystals, so the itc
> values will slowly drift apart (Linux discovers this from SAL and so
> doesn't bother to synchronize them).
>
> > Note that it is possible that this problem may exist for other ia64
> > machines as well, based on the following comment for sched_clock() in
> > arch/ia64/kernel/head.S:
> >
> > * Return a CPU-local timestamp in nano-seconds. This timestamp is
> > * NOT synchronized across CPUs its return value must never be
> > * compared against the values returned on another CPU. The usage in
> > * kernel/sched.c ensures that.
>
> When sched_clock() was first created this was part of its definition.
> It meant that sched_clock could be simple & fast because it didn't
> need to be synchronized across cpus.
>
> It seems that new uses have grown for it that no longer allow that
> flexibility.
>
> All ia64 systems are potentially affected ... but perhaps you might
> never see the problem on most because the itc clocks are synced as close
> as s/w can get them when cpus are brought on line.

Do you want Dimitri to resubmit with this set for all IA64 or leave it
as is?

Thanks,
Robin