linux-kernel - Re: [PATCH v2] sched/fair: update scale invariance of PELT

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Message-ID: <CAKfTPtDfcXBd7WzbHMBhUj381qh9JTuKz3Sv8cLj1MOZMo+9eg@mail.gmail.com>
Date:   Wed, 19 Apr 2017 18:31:00 +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>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Yuyang Du <yuyang.du@...el.com>, Paul Turner <pjt@...gle.com>,
        Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH v2] sched/fair: update scale invariance of PELT

On 14 April 2017 at 10:49, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> On 13 April 2017 at 18:13, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Thu, Apr 13, 2017 at 05:16:20PM +0200, Vincent Guittot wrote:
>>> On 13 April 2017 at 15:39, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>>> > OK, so the reason util_avg varies is because we compute it wrong. And I
>>> > think we can easily fix that once we pull out all the factors (which
>>> > would mean your patch and the pulling out of weight patch which still
>>> > needs to be finished).
>>>
>>> That would be great to remove this unwanted variation.
>>
>> So the problem with the _avg stuff is that we include the d3 segment,
>> that is the unfinished current window. Since we only re-compute the _avg
>> whenever we roll over, the intent already seems to be to only compute it
>> on completed windows.
>
> yes make sense
>
>>
>> But because 'complicated/expensive', its hard to not include d3 and thus
>> we get the wobble.
>>
>> Once we compute pure running/runnable sums, without extra contrib
>> factors, we can simply subtract our d3 term from sum when doing the
>> division and change the divider to LOAD_AVG_MAX*y, getting the stable
>> _avg over all completed windows.
>
> I'm going to make it a try to check that it removes the variation i'm seeing

I have sent a patchset based on your proposal that fix this variation issue

>
>>
>> (you could do the same with factors, but then we get to do a bunch of
>> extra multiplications which aren't free).
>>
>>> >
>>> > But you're comparing against util_sum here, that behaves slightly
>>> > different. I think you want 'util_sum >= 1024 * (LOAD_AVG_MAX - 1024)'
>>> > instead.
>>>
>>> yes, the variation happens on the util_sum
>>
>> Well, for util_sum its simple to ignore the current window, which is
>> what the suggested equation does (note that LOAD_AVG_MAX*y ==
>> LOAD_AVG_MAX-1024).