linux-kernel - Re: [RFC PATCH 1/2] sched/rt: add utilization tracking

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Message-ID: <CAKfTPtAE5XUrJaWPM-6j5tyJqdmOyFqBNrZ5VS2b6as0Vf2SMg@mail.gmail.com>
Date:   Wed, 31 May 2017 13:24:24 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        Juri Lelli <juri.lelli@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [RFC PATCH 1/2] sched/rt: add utilization tracking

On 31 May 2017 at 11:40, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, May 24, 2017 at 11:00:51AM +0200, Vincent Guittot wrote:
>> schedutil governor relies on cfs_rq's util_avg to choose the OPP when cfs
>> tasks are running. When the CPU is overloaded by cfs and rt tasks, cfs tasks
>> are preempted by rt tasks and in this case util_avg reflects the remaining
>> capacity that is used by cfs tasks but not what cfs tasks want to use. In such
>> case, schedutil can select a lower OPP when cfs task runs whereas the CPU is
>> overloaded. In order to have a more accurate view of the utilization of the
>> CPU, we track the utilization that is used by RT tasks.
>> DL tasks are not taken into account as they have their own utilization
>> tracking mecanism.
>
> Well, the DL tracking is fairly pessimistic; it assumes all DL tasks
> will consume their total budget, which will rarely, if ever, happen.
>
> So I suspect it might well be worth it to also track DL activity for the
> purpose of compensating CFS.
>
> In fact, I don't think you particularly care about RT here, as anything
> !CFS that preempts it, including those interrupts you mentioned. Which
> gets us back to what rt_avg is.
>
>> We don't use rt_avg which doesn't have the same dynamic as PELT and which
>> can include IRQ time that are also accounted in cfs task utilization
>
> Well, if rt_avg includes IRQ time, then that IRQ time is not part of
> the task clock.

ah yes you're right.
I haven't noticed irq time was removed from the clock used for accounting PELT

>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>>
>> If the changes are reasonnable, it might worth moving the PELT function in a
>> dedicated pelt.c file and the ugly
>> extern int update_rt_rq_load_avg(u64 now, int cpu, struct rt_rq *rt_rq, int running);
>> in a pelt.h header
>>
>>
>>  kernel/sched/fair.c  | 21 +++++++++++++++++++++
>>  kernel/sched/rt.c    |  9 +++++++++
>>  kernel/sched/sched.h |  3 +++
>>  3 files changed, 33 insertions(+)
>
> Also, and I didn't check this, it is important that the windows are
> aligned if you want to sum the values.

yes. good point