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

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Date:	Tue, 06 Jan 2009 21:57:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Robin Holt <holt@....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 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.
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