linux-kernel - Re: [PATCH v4 11/11] sched/fair: rework find_idlest

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Message-ID: <20191120195521.nvhzvd3x7l7jdxsr@e107158-lin.cambridge.arm.com>
Date:   Wed, 20 Nov 2019 19:55:22 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Hillf Danton <hdanton@...a.com>,
        Parth Shah <parth@...ux.ibm.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v4 11/11] sched/fair: rework find_idlest_group

On 11/20/19 20:28, Vincent Guittot wrote:
> I run few more tests and i can get either hw counter with 0 or not.
> The main difference is on which CPU it runs: either big or little
> little return always 0 and big always non-zero value
> 
> on v5.4-rc7 and tip/sched/core, cpu0-3 return 0 and other non zeroa
> but on next, it's the opposite cpu0-3 return non zero ratio
> 
> Could you try to run the test with taskset to run it on big or little ?

Nice catch!

Yes indeed using taskset and forcing it to run on the big cpus it passes even
on linux-next/next-20191119.

So the relation to your patch is that it just biased where this test is likely
to run in my case and highlighted the breakage in the counters, probably?

FWIW, if I use taskset to force always big it passes. Always small, the counters
are always 0 and it passes too. But if I have mixed I see what I pasted before,
the counters have valid value but nhw is 0.

So the questions are, why little counters aren't working. And whether we should
run the test with taskset generally as it can't handle the asymmetry correctly.

Let me first try to find out why the little counters aren't working.

Thanks

--
Qais Yousef