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[86.203.135.111]) by smtp.gmail.com with ESMTPSA id q11-20020adff50b000000b0031c71693449sm12572024wro.1.2023.09.18.05.22.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 18 Sep 2023 05:22:18 -0700 (PDT) From: Valentin Schneider To: 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, pierre.gondois@arm.com, 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 14:22:17 +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 23:40, Shrikanth Hegde wrote: > On 9/15/23 5:30 PM, 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). > > Yes. That's another way. Thats what I had as possible approach in > https://lore.kernel.org/lkml/d2c945d6-c4f0-a096-0623-731b11484f51@linux.vnet.ibm.com/ > Thanks for the link; and apologies for bringing up topics that have been discussed already. > > >> >> I can never remember how to properly use the sysctl API, so that's a very >> crude implementation, but something like so? >> >> --- >> >> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c >> index 05a5bc678c089..dadfc5afc4121 100644 >> --- a/kernel/sched/topology.c >> +++ b/kernel/sched/topology.c >> @@ -230,9 +230,28 @@ static int sched_energy_aware_handler(struct ctl_table *table, int write, >> if (write && !capable(CAP_SYS_ADMIN)) >> return -EPERM; >> >> + if (!sched_energy_enabled()) { > > Use of sched_energy_enabled won't work as Pierre has indicated. > > Instead this can be done by adding those checks in a helper function to > do similar checks as done build_perf_domains. > > I can send v4 with this approach if it makes more sense. Please let me know. > So what I'm thinking is the standard approach seems to be to keep the knobs visible, but change how reads/writes to them are handled. For instance, SMT support has /sys/devices/system/cpu/smt /control /active And a system with CONFIG_HOTPLUG_SMT=y but no actual hardware SMT will have: /control = notsupported /active = 0 So IMO it would make sense to keep sched_energy_aware around, but make it read 0 and prevent writes for systems that have the software support compiled but don't have the actual hardware support. In a pinch it also helps to know if CONFIG_ENERGY_MODEL was selected, though that's obvious enough with CONFIG_SCHED_DEBUG=y.