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[35.195.168.105]) by smtp.gmail.com with ESMTPSA id f8sm8337953wmc.8.2021.04.23.09.14.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Apr 2021 09:14:47 -0700 (PDT) Date: Fri, 23 Apr 2021 16:14:44 +0000 From: Quentin Perret To: Vincent Donnefort Cc: peterz@infradead.org, rjw@rjwysocki.net, viresh.kumar@linaro.org, vincent.guittot@linaro.org, linux-kernel@vger.kernel.org, ionela.voinescu@arm.com, lukasz.luba@arm.com, dietmar.eggemann@arm.com Subject: Re: [PATCH] PM / EM: Inefficient OPPs detection Message-ID: References: <1617901829-381963-1-git-send-email-vincent.donnefort@arm.com> <1617901829-381963-2-git-send-email-vincent.donnefort@arm.com> <20210422153644.GA316798@e124901.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210422153644.GA316798@e124901.cambridge.arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thursday 22 Apr 2021 at 16:36:44 (+0100), Vincent Donnefort wrote: > > > 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 > > On the Pixel4, I used rt-app to generate a task whom duty cycle is getting > higher for each phase. Then for each rt-app task placement, I measured how long > find_energy_efficient_cpu() took to run. I repeated the operation several > times to increase the count. Here's what I've got: > > ┌────────┬─────────────┬───────┬────────────────┬───────────────┬───────────────┐ > │ Phase │ duty-cycle │ CPU │ w/o LUT │ w/ LUT │ │ > │ │ │ ├────────┬───────┼───────┬───────┤ Diff │ > │ │ │ │ Mean │ count │ Mean │ count │ │ > ├────────┼─────────────┼───────┼────────┼───────┼───────┼───────┼───────────────┤ > │ 0 │ 12.5% │ Little│ 10791 │ 3124 │ 10657 │ 3741 │ -1.2% -134ns │ > ├────────┼─────────────┼───────┼────────┼───────┼───────┼───────┼───────────────┤ > │ 1 │ 25% │ Mid │ 2924 │ 3097 │ 2894 │ 3740 │ -1% -30ns │ > ├────────┼─────────────┼───────┼────────┼───────┼───────┼───────┼───────────────┤ > │ 2 │ 37.5% │ Mid │ 2207 │ 3104 │ 2162 │ 3740 │ -2% -45ns │ > ├────────┼─────────────┼───────┼────────┼───────┼───────┼───────┼───────────────┤ > │ 3 │ 50% │ Mid │ 1897 │ 3119 │ 1864 │ 3717 │ -1.7% -33ns │ > ├────────┼─────────────┼───────┼────────┼───────┼───────┼───────┼───────────────┤ > │ │ │ Mid │ 1700 │ 396 │ 1609 │ 1232 │ -5.4% -91ns │ > │ 4 │ 62.5% ├───────┼────────┼───────┼───────┼───────┼───────────────┤ > │ │ │ Big │ 1187 │ 2729 │ 1129 │ 2518 │ -4.9% -58ns │ > ├────────┼─────────────┼───────┼────────┼───────┼───────┼───────┼───────────────┤ > │ 5 │ 75% │ Big │ 984 │ 3124 │ 900 │ 3693 │ -8.5% -84ns │ > └────────┴─────────────┴───────┴────────┴───────┴───────┴───────┴───────────────┘ Thanks for that. Do you have the stddev handy? > Notice: > > * The CPU column describes which CPU ran the find_energy_efficient() > function. > > * I modified my patch so that no inefficient OPPs are reported. This is to > have a fairer comparison between the original table walk and the lookup > table. You mean to avoid the impact of the frequency selection itself? Maybe pinning the frequencies in the cpufreq policy could do? > > * I removed from the table results that didn't have enough count to be > statistically significant. Anyways, this looks like a small but consistent gain throughout, so it's a win for the LUT :) Thanks, Quentin