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

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Date:   Thu, 7 Jun 2018 10:29:15 +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 06/06/2018 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:
>>
>> [...]
>>
>>>>> +        */
>>>>> +       ret = ___update_load_sum(rq->clock - running, rq->cpu,
>>>>> &rq->avg_irq,
>>>>> +                               0,
>>>>> +                               0,
>>>>> +                               0);
>>>>> +       ret += ___update_load_sum(rq->clock, rq->cpu, &rq->avg_irq,
>>>>> +                               1,
>>>>> +                               1,
>>>>> +                               1);
>>
>> Can you not change the function parameter list to the usual
>> (u64 now, struct rq *rq, int running)?
>>
>> Something like this (only compile and boot tested):
> 
> To be honest, I prefer to keep the specific sequence above in a
> dedicated function instead of adding it in core code.

No big issue.

> Furthermore,  we end up calling call twice ___update_load_avg instead
> of only once. This will set an intermediate and incorrect value in
> util_avg and this value can be read in the meantime

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.

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?

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.

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