Re: [PATCH] PM / EM: Inefficient OPPs detection

[Lukasz Luba]

Hi Quentin,

On 4/15/21 4:20 PM, Quentin Perret wrote:
> On Thursday 15 Apr 2021 at 16:14:46 (+0100), Vincent Donnefort wrote:
> > On Thu, Apr 15, 2021 at 02:59:54PM +0000, Quentin Perret wrote:
> > > On Thursday 15 Apr 2021 at 15:34:53 (+0100), Vincent Donnefort wrote:
> > > > On Thu, Apr 15, 2021 at 01:16:35PM +0000, Quentin Perret wrote:
> > > > > On Thursday 08 Apr 2021 at 18:10:29 (+0100), Vincent Donnefort wrote:
> > > > > > --- a/kernel/sched/cpufreq_schedutil.c
> > > > > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > > > > @@ -10,6 +10,7 @@
> > > > > >   
> > > > > >   #include "sched.h"
> > > > > >   
> > > > > > +#include <linux/energy_model.h>
> > > > > >   #include <linux/sched/cpufreq.h>
> > > > > >   #include <trace/events/power.h>
> > > > > >   
> > > > > > @@ -164,6 +165,9 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> > > > > >   
> > > > > >   	freq = map_util_freq(util, freq, max);
> > > > > >   
> > > > > > +	/* Avoid inefficient performance states */
> > > > > > +	freq = em_pd_get_efficient_freq(em_cpu_get(policy->cpu), freq);
> > > > >
> > > > > I remember this was discussed when Douglas sent his patches some time
> > > > > ago, but I still find it sad we index the EM table here but still
> > > > > re-index the cpufreq frequency table later :/
> > > > >
> > > > > Yes in your case this lookup is very inexpensive, but still. EAS relies
> > > > > on the EM's table matching cpufreq's accurately, so this second lookup
> > > > > still feels rather unnecessary ...
> > > >
> > > > To get only a single lookup, we would need to bring the inefficiency knowledge
> > > > directly to the cpufreq framework. But it has its own limitations:
> > > >
> > > >    The cpufreq driver can have its own resolve_freq() callback, which means that
> > > >    not all the drivers would benefit from that feature.
> > > >
> > > >    The cpufreq_table can be ordered and accessed in several ways which brings
> > > >    many combinations that would need to be supported, ending-up with something
> > > >    much more intrusive. (We can though decide to limit the feature to the low to
> > > >    high access that schedutil needs).
> > > >
> > > > As the EM needs schedutil to exist anyway, it seemed to be the right place for
> > > > this code. It allows any cpufreq driver to benefit from the feature, simplify a
> > > > potential extension for a usage by devfreq devices and as a bonus it speeds-up
> > > > energy computing, allowing a more complex Energy Model.
> > >
> > > I was thinking of something a bit simpler. cpufreq_driver_resolve_freq
> > > appears to be used only from schedutil (why is it even then?), so we
> > > could just pull it into cpufreq_schedutil.c and just plain skip the call
> > > to cpufreq_frequency_table_target if the target freq has been indexed in
> > > the EM table -- it should already be matching a real OPP.
> > >
> > > Thoughts?
> > > Quentin
> >
> > Can try that for a V2. That means em_pd_get_efficient_freq() would have to
> > know about policy clamping (but I don't think that's an issue)
>
> Indeed, and I think we can even see this as an improvement as EAS will
> now see policy clamps as well in compute_energy().

Are you sure that the 'policy' can be accessed from compute_energy()?
It can be from schedutil freq switch path, but I'm not use about our
feec()..

For me this cpufreq_driver_resolve_freq sounds a bit out of this patch
subject.

Regards,
Lukasz