linux-kernel - Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI

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Message-ID: <20090106201950.GA3850@sgi.com>
Date:	Tue, 6 Jan 2009 14:19:50 -0600
From:	Robin Holt <holt@....com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Dimitri Sivanich <sivanich@....com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Greg KH <greg@...ah.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Nick Piggin <npiggin@...e.de>, Tony Luck <tony.luck@...il.com>,
	Robin Holt <holt@....com>
Subject: Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems

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
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