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

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Date:   Thu, 21 Nov 2019 14:58:09 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mark.rutland@....com
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 19:55, Qais Yousef wrote:
> 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.

So it turns out there's a caveat on usage of perf counters on big.LITTLE
systems.

Mark on CC can explain this better than me so I'll leave the details to him.

Sorry about the noise Vincent - it seems your patch was shifting things
slightly to cause migrating the task to another CPU, hence trigger the failure
on reading the perf counters, and the test in return.

Thanks

--
Qais Yousef