linux-kernel - Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

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Message-ID: <94eeb587-e9c9-1d92-7fd3-edde46fd4dba@arm.com>
Date:   Wed, 22 Feb 2023 21:28:54 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Qais Yousef <qyousef@...alina.io>,
        Kajetan Puchalski <kajetan.puchalski@....com>,
        Jian-Min Liu <jian-min.liu@...iatek.com>,
        Ingo Molnar <mingo@...nel.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel@...r.kernel.org,
        Jonathan JMChen <jonathan.jmchen@...iatek.com>
Subject: Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

On 21/02/2023 10:29, Vincent Guittot wrote:
> On Mon, 20 Feb 2023 at 14:54, Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
>>
>> On Fri, 17 Feb 2023 at 14:54, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>
>>> On 09/02/2023 17:16, Vincent Guittot wrote:
>>>> On Tue, 7 Feb 2023 at 11:29, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>>
>>>>> On 09/11/2022 16:49, Peter Zijlstra wrote:
>>>>>> On Tue, Nov 08, 2022 at 07:48:43PM +0000, Qais Yousef wrote:
>>>>>>> On 11/07/22 14:41, Peter Zijlstra wrote:
>>>>>>>> On Thu, Sep 29, 2022 at 03:41:47PM +0100, Kajetan Puchalski wrote:

[...]

>>> I ran the same test (boosting only for DVFS requests) with:
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ *
>>>
>>> -->8--
>>>
>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>>> index dbc56e8b85f9..7a4bf38f2920 100644
>>> --- a/kernel/sched/sched.h
>>> +++ b/kernel/sched/sched.h
>>> @@ -2946,6 +2946,8 @@ static inline unsigned long cpu_util_cfs(int cpu)
>>>                              READ_ONCE(cfs_rq->avg.util_est.enqueued));
>>>         }
>>>
>>> +       util = max(util, READ_ONCE(cfs_rq->avg.runnable_avg));
>>> +
> 
> Another reason why it gives better results could be that
> cpu_util_cfs() is not only used for DVFS selection but also to track
> the cpu utilization in load balance and EAS so the cpu will be faster
> seen as overloaded and tasks will be spread around when there are
> contentions.
> 
> Could you try to take cfs_rq->avg.runnable_avg into account only when
> selecting frequency ?

I actually did exactly this. (* but not shown in the code snippet).
I just used the boosting for CPU frequency selection (from
sugov_get_util()). I added the the `_freq` suffix in the kernel name to
indicate this.

> That being said I can see some place in load balance where
> cfs_rq->avg.runnable_avg could give some benefits like in
> find_busiest_queue() where it could be better to take into account the
> contention when selecting the busiest queue

Could be. Looks like so far we only use it in group_has_capacity(),
group_is_overloaded() and for NUMA.

[...]