linux-kernel - Re: [PATCH] cros_ec_spi: Even though we're RT priority, don't bump cpu freq

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Date:   Wed, 24 Jun 2020 13:55:02 -0400
From:   Joel Fernandes <joelaf@...gle.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Doug Anderson <dianders@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Benson Leung <bleung@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        hsinyi@...omium.org, Nicolas Boichat <drinkcat@...omium.org>,
        Gwendal Grignou <gwendal@...omium.org>,
        Quentin Perret <qperret@...gle.com>, ctheegal@...eaurora.org,
        Guenter Roeck <groeck@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cros_ec_spi: Even though we're RT priority, don't bump
 cpu freq

On Wed, Jun 24, 2020 at 1:52 PM Qais Yousef <qais.yousef@....com> wrote:
>
> On 06/24/20 13:35, Joel Fernandes wrote:
>
> [...]
>
> > > Doing the in-kernel opt-out via API should be fine, I think. But this will
> > > need to be discussed in the wider circle. It will already clash with this for
> > > example
> > >
> > > https://lore.kernel.org/lkml/20200619172011.5810-1-qais.yousef@arm.com/
> >
> > Have not yet looked closer at that patch, but are you saying this
> > patch clashes with that work? Sorry I am operating on 2 hours of sleep
> > here.
>
> The series is an optimization to remove the uclamp overhead from the scheduler
> fastpath until the userspace uses it. It introduces a static key that is
> disabled by default and will cause uclamp logic not to execute in the fast
> path. Once the userspace starts using util clamp, which we detect by either
>
>         1. Changing uclamp value of a task with sched_setattr()
>         2. Modifying the default sysctl_sched_util_clamp_{min, max}
>         3. Modifying the default cpu.uclamp.{min, max} value in cgroup
>
> If we start having in-kernel users changing uclamp value this means drivers
> will cause the system to opt-in into uclamp automatically even if the
> userspace doesn't actually use it.
>
> I think we can solve this by providing a special API to opt-out safely. Which
> is the right thing to do anyway even if we didn't have this clash.

Makes sense, thanks.

 - Joel