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Date:   Tue, 19 Jan 2021 16:40:27 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     Quentin Perret <qperret@google.com>
Cc:     "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        linux-kernel@vger.kernel.org,
        Valentin Schneider <valentin.schneider@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>
Subject: Re: [PATCH] sched/eas: Don't update misfit status if the task is
 pinned
Message-ID: <20210119164027.drfpmrol3xhf4ckc@e107158-lin>
References: <20210119120755.2425264-1-qais.yousef@arm.com>
 <YAb8XGyp3NtrHl+U@google.com>
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On 01/19/21 15:35, Quentin Perret wrote:
> On Tuesday 19 Jan 2021 at 12:07:55 (+0000), Qais Yousef wrote:
> > If the task is pinned to a cpu, setting the misfit status means that
> > we'll unnecessarily continuously attempt to migrate the task but fail.
> > 
> > This continuous failure will cause the balance_interval to increase to
> > a high value, and eventually cause unnecessary significant delays in
> > balancing the system when real imbalance happens.
> > 
> > Caught while testing uclamp where rt-app calibration loop was pinned to
> > cpu 0, shortly after which we spawn another task with high util_clamp
> > value. The task was failing to migrate after over 40ms of runtime due to
> > balance_interval unnecessary expanded to a very high value from the
> > calibration loop.
> > 
> > Not done here, but it could be useful to extend the check for pinning to
> > verify that the affinity of the task has a cpu that fits. We could end
> > up in a similar situation otherwise.
> 
> Do you mean failing the sched_setaffinity syscall if e.g. the task
> has a min clamp that is higher than the capacity of the CPUs to which it
> will be pinned? If so, I'm not sure if we really want that.

No. In Android for instance, I'm worried a background task affined to little
cores that has a utilization > capacity_of(little) will trigger the same
problem. It'll be affined to more than just 1 cpu, but none of the little cpus
will actually fit.

Makes sense?

> But this patch makes sense on its own for sure, so:
> 
> Reviewed-by: Quentin Perret <qperret@google.com>

Thanks

--
Qais Yousef