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

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Message-ID: <20090107094328.GD3850@sgi.com>
Date:	Wed, 7 Jan 2009 03:43:28 -0600
From:	Robin Holt <holt@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Nick Piggin <npiggin@...e.de>, Robin Holt <holt@....com>,
	"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>,
	Tony Luck <tony.luck@...il.com>
Subject: Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems

On Wed, Jan 07, 2009 at 08:28:09AM +0100, Peter Zijlstra wrote:
> That's basically what the HAVE_UNSTABLE_SCHED_CLOCK code does. It takes
> a tick timestamp and tries to improve on that by using strict per cpu
> sched_clock() deltas.
> 
> What we do to obtain remote time, is basically calculate local time and
> pull remote time fwd if that was behind.

But what happens when the remote clock then takes a single tick and
pulls forward a different remote clock by both your tick and its tick.
Multiply that by 4096 and I am starting to feel that time is likely to
go racing forward in a really random fashion.

> While doing that, it filters out any backward motion and large fwd leaps
> so as to stay no worse than a jiffie clock.
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