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

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Date:	Thu, 9 Dec 2010 14:21:33 -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 10:55 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2010-12-09 at 10:11 -0800, Venkatesh Pallipadi 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
>
> Ah, something like that would happen when we do a wakeup from
> soft/hard-irq context, not an altogether uncommon occurrence.
>
> Wouldn't that be cured by updating the irq-time when asking for it,
> something like the below? (on top of my earlier patch)

This should mostly work. There would be a small window there on
rq->clock=sched_clock_cpu() and system_vtime doing sched_clock_cpu()
and also the overhead of doing this everytime when this going
backwards may happen rarely.

Thanks,
Venki

>
> ---
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -1843,8 +1843,9 @@ void account_system_vtime(struct task_st
>  }
>  EXPORT_SYMBOL_GPL(account_system_vtime);
>
> -static inline u64 irq_time_cpu(int cpu)
> +static u64 irq_time_cpu(int cpu)
>  {
> +       account_system_vtime(current);
>        return per_cpu(cpu_softirq_time, cpu) + per_cpu(cpu_hardirq_time, cpu);
>  }
>
>
>
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