Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to related_cpus unnecessarily"

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, May 08, 2018 at 10:42:37AM +0100, Quentin Perret wrote:
> On Tuesday 08 May 2018 at 11:09:57 (+0200), Dietmar Eggemann wrote:
> > On 05/08/2018 10:22 AM, Viresh Kumar wrote:
> > > On 08-05-18, 08:33, Dietmar Eggemann wrote:
> > > > This reverts commit e2cabe48c20efb174ce0c01190f8b9c5f3ea1d13.
> > > > 
> > > > Lifting the restriction that the sugov kthread is bound to the
> > > > policy->related_cpus for a system with a slow switching cpufreq driver,
> > > > which is able to perform DVFS from any cpu (e.g. cpufreq-dt), is not
> > > > only not beneficial it also harms Enery-Aware Scheduling (EAS) on
> > > > systems with asymmetric cpu capacities (e.g. Arm big.LITTLE).
> > > > 
> > > > The sugov kthread which does the update for the little cpus could
> > > > potentially run on a big cpu. It could prevent that the big cluster goes
> > > > into deeper idle states although all the tasks are running on the little
> > > > cluster.
> > > 
> > > I think the original patch did the right thing, but that doesn't suit
> > > everybody as you explained.
> > > 
> > > I wouldn't really revert the patch but fix my platform's cpufreq
> > > driver to set dvfs_possible_from_any_cpu = false, so that other
> > > platforms can still benefit from the original commit.
> > 
> > This would make sure that the kthreads are bound to the correct set of cpus
> > for platforms with those cpufreq drivers (cpufreq-dt (h960), scmi-cpufreq,
> > scpi-cpufreq) but it will also change the logic (e.g.
> > sugov_should_update_freq() -> cpufreq_can_do_remote_dvfs()).
> > 
> > I'm still struggling to understand when a driver/platform should set
> > dvfs_possible_from_any_cpu to true and what the actual benefit would be.
> 
> I assume it might be beneficial to have the kthread moving around freely
> in some cases, but since it is a SCHED_DEADLINE task now it can't really
> migrate anywhere anyway. So I'm not sure either if this commits still makes
> sense now. Or is there another use case for this ?

The usecase I guess is, as Dietmar was saying, that it makes sense for
kthread to update its own cluster and not disturb other clusters or random
CPUs. I agree with this point.

- Joel