linux-kernel - Re: [PATCH v2 3/3] sched/fair: fix unnecessary increase of balance interval

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Message-ID: <6afedeb0-62bc-27ec-56f0-107be22d2494@arm.com>
Date:   Wed, 19 Dec 2018 15:54:31 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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 v2 3/3] sched/fair: fix unnecessary increase of balance
 interval

On 19/12/2018 13:29, Vincent Guittot wrote:
[...]
>> My point is that AFAICT the LBF_ALL_PINNED flag would cover all the cases
>> we care about, although the one you're mentioning is the only one I can
>> think of. In that case LBF_ALL_PINNED would never be cleared, so when we do
>> the active balance we'd know it's because all other tasks were pinned so
>> we should probably increase the interval (see last snippet I sent).
> 
> There are probably several other UC than the one mentioned below as
> tasks can be discarded for several reasons.
> So instead of changing for all UC by default, i would prefer only
> change for those for which we know it's safe

I get your point. Thing is, I've stared at the code for a while and
couldn't find any other usecase where checking LBF_ALL_PINNED wouldn't
suffice.

It would be nice convince ourselves it is indeed enough (or not, but then
we should be sure of it rather than base ourselves on assumptions), because
then we can have just a simple condition rather than something that
introduces active balance categories.