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[86.203.135.111]) by smtp.gmail.com with ESMTPSA id f4-20020adfdb44000000b00318147fd2d3sm12653529wrj.41.2023.09.18.05.21.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 18 Sep 2023 05:21:02 -0700 (PDT) From: Valentin Schneider To: Pierre Gondois , Shrikanth Hegde Cc: dietmar.eggemann@arm.com, linux-kernel@vger.kernel.org, ionela.voinescu@arm.com, quentin.perret@arm.com, srikar@linux.vnet.ibm.com, mgorman@techsingularity.net, mingo@kernel.org, yu.c.chen@intel.com, tim.c.chen@linux.intel.com, mingo@redhat.com, peterz@infradead.org, vincent.guittot@linaro.org Subject: Re: [PATCH v3] sched/topology: remove sysctl_sched_energy_aware depending on the architecture In-Reply-To: References: <20230913114807.665094-1-sshegde@linux.vnet.ibm.com> <7c6dd2ec-b9a9-b364-5a29-05336127e519@linux.vnet.ibm.com> Date: Mon, 18 Sep 2023 14:21:01 +0200 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/09/23 15:35, Pierre Gondois wrote: > Hello Valentin, > > On 9/15/23 14:00, Valentin Schneider wrote: >> On 14/09/23 23:26, Shrikanth Hegde wrote: >>> On 9/14/23 9:51 PM, Valentin Schneider wrote: >>>> On 13/09/23 17:18, Shrikanth Hegde wrote: >>>>> sysctl_sched_energy_aware is available for the admin to disable/enable >>>>> energy aware scheduling(EAS). EAS is enabled only if few conditions are >>>>> met by the platform. They are, asymmetric CPU capacity, no SMT, >>>>> valid cpufreq policy, frequency invariant load tracking. It is possible >>>>> platform when booting may not have EAS capability, but can do that after. >>>>> For example, changing/registering the cpufreq policy. >>>>> >>>>> At present, though platform doesn't support EAS, this sysctl is still >>>>> present and it ends up calling rebuild of sched domain on write to 1 and >>>>> NOP when writing to 0. That is confusing and un-necessary. >>>>> >>>> >>> >>> Hi Valentin, Thanks for taking a look at this patch. >>> >>>> But why would you write to it in the first place? Or do you mean to use >>>> this as an indicator for userspace that EAS is supported? >>>> >>> >>> Since this sysctl is present and its value being 1, it gives the >>> impression to the user that EAS is supported when it is not. >>> So its an attempt to correct that part. >>> >> >> Ah, I see. Then how about just making the sysctl return 0 when EAS isn't >> supported? And on top of it, prevent all writes when EAS isn't supported >> (perf domains cannot be built, so there would be no point in forcing a >> rebuild that will do nothing). > > I think the issue comes from the fact there is no variable representing > whether EAS is supported or not. sched_energy_enabled()/sched_energy_present > tells whether EAS is actively running on the system instead. > > So on a system with EAS running, I think what would happen is: > # Disable EAS and set sched_energy_present=0 > echo 0 > /proc/sys/kernel/sched_energy_aware > > # sched_energy_present==0, so we get -EOPNOTSUPP > echo 1 > /proc/sys/kernel/sched_energy_aware > Ah, quite so, I didn't think this through!