linux-kernel - Re: [BUG] Paravirtual time accounting / IRQ time accounting

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Date:	Fri, 21 Mar 2014 07:31:58 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Glauber Costa <glommer@...il.com>, lwcheng@...hku.hk,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] Paravirtual time accounting / IRQ time accounting

On 03/20/2014 11:01 AM, Glauber Costa wrote:
> On Wed, Mar 19, 2014 at 6:42 AM,  <lwcheng@...hku.hk> wrote:

>> ------------
>> [src/kernel/sched/core.c]
>> static void update_rq_clock_task(struct rq *rq, s64 delta)
>> {
>>     ... ...
>> #ifdef CONFIG_IRQ_TIME_ACCOUNTING
>>     irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
>>     ... ...
>>     rq->prev_irq_time += irq_delta;
>>     delta -= irq_delta;
>> #endif
>>
>> #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
>>     if (static_key_false((&paravirt_steal_rq_enabled))) {
>>         steal = paravirt_steal_clock(cpu_of(rq));
>>         steal -= rq->prev_steal_time_rq;
>>         ... ...
>>         rq->prev_steal_time_rq += steal;
>>         delta -= steal;
>>     }
>> #endif
>>
>>     rq->clock_task += delta;
>>     ... ...
>> }
>> --
>> "delta" -> the intended increment to rq->clock_task
>> "irq_delta" -> the time spent on serving IRQ (hard + soft)
>> "steal" -> the time stolen by the underlying hypervisor
>> --
>> "irq_delta" is calculated based on sched_clock_cpu(), which is vulnerable
>> to VM scheduling delays.
> 
> This looks like a real problem indeed. The main problem in searching
> for a solution, is that of course not all of the irq time is steal
> time and vice versa. In this case, we could subtract irq_time from
> steal, and add only the steal part time that is in excess. I don't
> think this is 100 % guaranteed, but maybe it is a good approximation.
> 
> Rik, do you have an opinion on this ?

The other way around may be better, since steal time (when it
happens) is likely to be of "time slice" magnitude, while irq
time will happen more frequently, and in dozens-of-microseconds
intervals.

Furthermore, we have no way to measure what the irq time is,
except by looking at how much real time elapsed. For steal time,
however, the hypervisor tells us exactly how much time was stolen.

That means when steal time and irq time happen simultaneously,
the amount of steal time should always be smaller than the
calculated irq time for that period.

actual irq_time = calculated irq time - reported steal time;

-- 
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