linux-kernel - Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

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Message-ID: <20101210165021.GA28263@n2100.arm.linux.org.uk>
Date:	Fri, 10 Dec 2010 16:50:21 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Venkatesh Pallipadi <venki@...gle.com>,
	Mikael Pettersson <mikpe@...uu.se>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

On Fri, Dec 10, 2010 at 02:47:46PM +0100, Peter Zijlstra wrote:
> 
> Full patch..
> 
> ---
> Subject: sched: Fix the irqtime code to deal with u64 wraps
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Thu Dec 09 14:15:34 CET 2010
> 
> ARM systems have a 32bit sched_clock() [ which needs to be fixed ],
> but this exposed a bug in the irq_time code as well, it doesn't deal
> with wraps at all.
> 
> Fix the irq_time code to deal with u64 wraps by re-writing the code to
> only use delta increments, which avoids the whole issue.
> 
> Furthermore, solve the problem of 32bit arches reading partial updates
> of the u64 time values.
> 
> Cc: Venkatesh Pallipadi <venki@...gle.com>
> Reported-by: Russell King - ARM Linux <linux@....linux.org.uk>

I think credit should go to Mikael Pettersson, who identified the
interactivity regression and problematical commit.  I only pointed
out the dubious nature of the code.
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