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

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Date:	Mon, 13 Dec 2010 20:03:02 +0530
From:	Jack Daniel <wanders.thirst@...il.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 11:41 PM, Venkatesh Pallipadi <venki@...gle.com> wrote:
> 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
                                  ^^^
I was curious on this line. Shouldn't this be irq_delta ~= t3 - t1 ?
If so the time going backwards shouldn't happen. If it does happen
then wouldn't it be better to program irq_time_cpu(cpu_of(rq)) to
return t3 instead of t4?

Thanks,
Jack
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