linux-kernel - Re: [PATCH] PM / EM: Inefficient OPPs detection

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Date:   Thu, 15 Apr 2021 15:12:08 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     peterz@...radead.org, rjw@...ysocki.net, viresh.kumar@...aro.org,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        ionela.voinescu@....com, lukasz.luba@....com,
        dietmar.eggemann@....com
Subject: Re: [PATCH] PM / EM: Inefficient OPPs detection

On Thu, Apr 15, 2021 at 01:12:05PM +0000, Quentin Perret wrote:
> Hi Vincent,
> 
> On Thursday 08 Apr 2021 at 18:10:29 (+0100), Vincent Donnefort wrote:
> > Some SoCs, such as the sd855 have OPPs within the same performance domain,
> > whose cost is higher than others with a higher frequency. Even though
> > those OPPs are interesting from a cooling perspective, it makes no sense
> > to use them when the device can run at full capacity. Those OPPs handicap
> > the performance domain, when choosing the most energy-efficient CPU and
> > are wasting energy. They are inefficient.
> > 
> > Hence, add support for such OPPs to the Energy Model, which creates for
> > each OPP a performance state. The Energy Model can now be read using the
> > regular table, which contains all performance states available, or using
> > an efficient table, where inefficient performance states (and by
> > extension, inefficient OPPs) have been removed.
> > 
> > Currently, the efficient table is used in two paths. Schedutil, and
> > find_energy_efficient_cpu(). We have to modify both paths in the same
> > patch so they stay synchronized. The thermal framework still relies on
> > the original table and hence, DevFreq devices won't create the efficient
> > table.
> > 
> > As used in the hot-path, the efficient table is a lookup table, generated
> > dynamically when the perf domain is created. The complexity of searching
> > a performance state is hence changed from O(n) to O(1). This also
> > speeds-up em_cpu_energy() even if no inefficient OPPs have been found.
> 
> Interesting. Do you have measurements showing the benefits on wake-up
> duration? I remember doing so by hacking the wake-up path to force tasks
> into feec()/compute_energy() even when overutilized, and then running
> hackbench. Maybe something like that would work for you?

I'll give a try and see if I get improved numbers.

> 
> Just want to make sure we actually need all that complexity -- while
> it's good to reduce the asymptotic complexity, we're looking at a rather
> small problem (max 30 OPPs or so I expect?), so other effects may be
> dominating. Simply skipping inefficient OPPs could be implemented in a
> much simpler way I think.

I could indeed just skip the perf state if marked as ineffective. But the idea
was to avoid bringing another for loop in this hot-path.

Also, not covered by this patch but probably we could get rid of the EM
complexity limit as the table resolution is way faster with this change.

> 
> Thanks,
> Quentin