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

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Date:	Thu, 9 Dec 2010 10:11:33 -0800
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	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 Thu, Dec 9, 2010 at 9:55 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2010-12-09 at 09:43 -0800, Venkatesh Pallipadi wrote:
>>
>> The same problem will be there with below code, with irq_delta >
>> delta, clock_task can go backwards which is not good.
>> +       delta -= irq_delta;
>> +       rq->clock_task += delta;
>>
>> The reason for this is rq->clock and irqtime updates kind of happen
>> independently and specifically, if a rq->clock update happens while we
>> are in a softirq, we may have this case of going backwards on the next
>> update.
>
> But how can irq_delta > delta?, we measure it using the same clock.
>

This would be mostly a corner case like:
- softirq start time t1
- rq->clock updated at t2 and rq->clock_task updated at t2 without
accounting for current softirq
- softirq end time t3
- cpu spends most time here in softirq or hardirq
- next rq->clock update at t4 and rq->clock_task update, with delta =
t4-t2 and irq_delta ~= t4 - t1
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