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

[Joel Fernandes <joelaf@google.com>]

On Wed, Jun 24, 2020 at 1:52 PM Qais Yousef <qais.yousef@arm.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