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

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Date:   Thu, 15 Apr 2021 13:12:05 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Vincent Donnefort <vincent.donnefort@....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

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?

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.

Thanks,
Quentin