Messages in this thread | | | Date | Thu, 15 Apr 2021 13:12:05 +0000 | From | Quentin Perret <> | Subject | Re: [PATCH] PM / EM: Inefficient OPPs detection |
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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
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