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

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Message-Id: <1231283763.11687.135.camel@twins>
Date:	Wed, 07 Jan 2009 00:16:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Robin Holt <holt@....com>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	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>,
	Gregory Haskins <ghaskins@...ell.com>,
	Nick Piggin <npiggin@...e.de>, Tony Luck <tony.luck@...il.com>
Subject: Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems

On Tue, 2009-01-06 at 16:50 -0600, Robin Holt wrote:
> On Tue, Jan 06, 2009 at 09:57:20PM +0100, Peter Zijlstra wrote:
> > On Tue, 2009-01-06 at 12:34 -0800, Luck, Tony wrote:
> > > > > 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?
> > > 
> > > I'd like to understand the impact of turning on HAVE_UNSTABLE_SCHED_CLOCK
> > > 
> > > It looks like both the i386_defconfig and x86_64_defconfig choose this,
> > > so at least ia64 will be hitting the well tested code paths
> > > 
> > > Have the other architectures just not hit this yet?  Or do they all have
> > > "stable" sched_clock() functions?
> > > 
> > > 
> > > sched_clock() seemed like such a straightforward thing to begin with. A
> > > quick & easy way to measure a time delta ON THE SAME CPU.  I'm not at
> > > all sure why it has been co-opted for general time measurement.
> > 
> > It came from the complication of needing to tell a remote cpu's time due
> > to remote wakeups in the scheduler.
> 
> But doesn't scheduler tick advance the rq->clock?  Why do the others
> need to fiddle with a remote runqueue's clock?  When that cpu starts
> taking ticks again, it will update it's rq->clock field and start the
> processes.  I guess I am a lot underinformed about the new scheduler
> design.

We try to do better than tick based time accounting these days.
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