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

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Date:	Wed, 07 Jan 2009 16:16:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dimitri Sivanich <sivanich@....com>
Cc:	Robin Holt <holt@....com>, Nick Piggin <npiggin@...e.de>,
	"Luck, Tony" <tony.luck@...el.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>,
	Tony Luck <tony.luck@...il.com>
Subject: Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems

On Wed, 2009-01-07 at 07:32 -0600, Dimitri Sivanich wrote:
> Peter,
> 
> On Wed, Jan 07, 2009 at 10:53:57AM +0100, Peter Zijlstra wrote:
> > 
> > Clock state is kept per-cpu, and locked with a spinlock. When we request
> 
> Admittedly I have not looked at this possibility too closely, but my
> initial concern upon looking at sched_clock_cpu() for the UNSTABLE
> case was the lock_double_clock() and what sort of contention that
> might cause on larger systems under certain conditions.

Similar contention would already exist on rq->lock.
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