linux-kernel - Re: [PATCH 3/3] sched/fair: fix unnecessary increase of balance interval

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Date:   Fri, 21 Dec 2018 18:41:39 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH 3/3] sched/fair: fix unnecessary increase of balance interval

On Fri, 21 Dec 2018 at 18:15, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 21/12/2018 14:49, Vincent Guittot wrote:
> [...]
> > After looking at shed.c at this sha1,  (sd->nr_balance_failed >
> > sd->cache_nice_tries+2) was the only condition for doing active
> > migration and as a result it was the only reason for doubling
> > sd->balance_interval.
> > My patch keeps exactly the same behavior for this condition
> > 'sd->nr_balance_failed > sd->cache_nice_tries+2). And, I'm even more
> > convinced to exclude (sd->nr_balance_failed > sd->cache_nice_tries+2)
> > condition because it's the condition that has introduced the doubling
> > of the interval.
> >
> > As said previously, you can argue that this behavior is not optimal
> > and discuss its validity, but the sha1 that you mentioned above,
> > introduced the current policy for (sd->nr_balance_failed >
> > sd->cache_nice_tries+2) condition.
> > Reverting such behavior would need more studies, tests and cares
>
> I agree with you on that, those are valid concerns.
>
> What I'm arguing for is instead of doing this in two steps (reset interval
> only for some active balance types, then experiment only increasing it for
> "active balance as a last resort"), I'd prefer doing it in one step.

Doing in 2 steps has the advantage of not delaying the current fix and
gives enough time for a complete study on the other step

>
> Mostly because I think the intermediate step adds an active balance
> categorization that can easily become confusing. Furthermore, that 2005
> commit explicitly states it wants to cater to pinned tasks, but we didn't
> have those LBF_* flags back then, so if we are to do something about it
> we should be improving upon the original intent.
>
> In the end it's not for me to decide, I just happen to find doing it that
> way more elegant (from a functionality & code readability PoV).
>
> > which
> > are out of the scope of this patchset and more related to a whole
> > refactoring of load_balance and calculte_imbalance; FYI, I have
> > submitted a topic on the subject for the next OSPM
>