linux-kernel - Re: [PATCHv4 12/12] sched/core: Disable SD_PREFER_SIBLING on asymmetric cpu capacity domains

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Date:   Mon, 6 Aug 2018 11:53:50 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Morten Rasmussen <morten.rasmussen@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        gaku.inami.xh@...esas.com,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv4 12/12] sched/core: Disable SD_PREFER_SIBLING on
 asymmetric cpu capacity domains

Hi,

On 06/08/18 11:20, Vincent Guittot wrote:
> Hi Valentin,
> 
> On Tue, 31 Jul 2018 at 14:33, Valentin Schneider
> <valentin.schneider@....com> wrote:
>>
>> Hi,
>>
>> On 31/07/18 13:17, Vincent Guittot wrote:
>> [...]
>>>>
>>>> This can easily happen with SD_PREFER_SIBLING enabled too so I wouldn't
>>>> say that this patch breaks anything that isn't broken already. In fact
>>>> we this happening with and without this patch applied.
>>>
>>> At least for the use case  above, this doesn't happen when
>>> SD_PREFER_SIBLING is set
>>>
>>
>> On my HiKey960 I can see coscheduling on a big CPU while a LITTLE is free
>> with **and** without SD_PREFER_SIBLING. Having it set only means that in
>> some cases the imbalance will be re-classified as group_overloaded instead
>> of group_misfit_task, so we'll skip the misfit logic when we shouldn't (this
>> happens on Juno for instance).
> 
> Can you give more details about your test case ?
> 

I've been running the same test case as presented in the cover letter on
my HiKey960 but with sched_switch tracing and with no tasksets. I've just
re-run the testcase with tasksets and I get similar results (i.e. a big
with coscheduling while a LITTLE is free) with or without the flag.

>>
>> It does nothing for the "1 task per CPU" problem that Morten described above.
>> When you have this little amount of tasks, load isn't very relevant, but it
>> skews the load-balancer into thinking the LITTLE CPUs are more busy than
>> the bigs even though there's an idle one in the lot.
>>
>>>>
>>>> Morten