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

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Date:   Fri, 14 Apr 2017 10:49:36 +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 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

>
> (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).