linux-kernel - Re: [PATCH v5 07/10] sched/irq: add irq utilization tracking

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Date:   Thu, 7 Jun 2018 11:06:29 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Quentin Perret <quentin.perret@....com>
Subject: Re: [PATCH v5 07/10] sched/irq: add irq utilization tracking

On 06/07/2018 10:44 AM, Vincent Guittot wrote:
> On 7 June 2018 at 10:29, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>> On 06/06/2018 06:06 PM, Vincent Guittot wrote:
>>>
>>> Hi Dietmar,
>>>
>>> Sorry for the late answer
>>>
>>> On 31 May 2018 at 18:54, Dietmar Eggemann <dietmar.eggemann@....com>
>>> wrote:
>>>>
>>>> On 05/30/2018 08:45 PM, Vincent Guittot wrote:
>>>>>
>>>>> Hi Dietmar,
>>>>>
>>>>> On 30 May 2018 at 17:55, Dietmar Eggemann <dietmar.eggemann@....com>
>>>>> wrote:
>>>>>>
>>>>>> On 05/25/2018 03:12 PM, Vincent Guittot wrote:

[...]

>> Can't buy this argument though because this is true with the current
>> implementation as well since the 'decay load sum' - 'accrue load sum'
>> sequence is not atomic.
> 
> it's not a problem that the _sum variable are updated in different
> step because there are internal variable
> Only util_avg is used "outside" and the latter is updated after both
> idle and running steps have been applied

You're right here!

>> What about calling update_irq_load_avg(rq, 0) in update_rq_clock_task() if
>> (irq_delta + steal) eq. 0 and sched_feat(NONTASK_CAPACITY) eq. true in this
>> #ifdef CONFIG_XXX_TIME_ACCOUNTING block?
> 
> update_irq_load_avg(rq, 0) is called in update_blocked_averages to
> decay smoothly like other blocked signals and replace the need to call
> update_irq_load_avg(rq, 0) for every call to update_rq_clock_task()
> which can be significant

OK.

>> Maintaining a irq/steal time signal makes only sense if at least one of the
>> CONFIG_XXX_TIME_ACCOUNTING is set and NONTASK_CAPACITY is true. The call to
>> update_irq_load_avg() in update_blocked_averages() isn't guarded my them.
> 
> good point

[...]