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    SubjectRe: [PATCH] PM / EM: Inefficient OPPs detection
    On Thursday 15 Apr 2021 at 15:12:08 (+0100), Vincent Donnefort wrote:
    > 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.

    Right, though it would just extend a little bit the existing loop, so
    the overhead is unlikely to be noticeable.

    > 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.

    Probably yeah. I was considering removing it since eb92692b2544
    ("sched/fair: Speed-up energy-aware wake-ups") but ended up keeping it
    as it's entirely untested on large systems. But maybe we can reconsider.

    Thanks,
    Quentin

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