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Mon, 18 Sep 2023 17:39:15 +0000 (GMT) Message-ID: <5a2e835e-557b-f87c-b2f0-25670c70e9ad@linux.vnet.ibm.com> Date: Mon, 18 Sep 2023 23:09:13 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v3] sched/topology: remove sysctl_sched_energy_aware depending on the architecture Content-Language: en-US To: Valentin Schneider , pierre.gondois@arm.com, pauld@redhat.com 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 References: <20230913114807.665094-1-sshegde@linux.vnet.ibm.com> <7c6dd2ec-b9a9-b364-5a29-05336127e519@linux.vnet.ibm.com> From: Shrikanth Hegde In-Reply-To: Content-Type: text/plain; charset=UTF-8 X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: NXy51xejNI_ZO9fTuFpC1DB-2QLZd2SO X-Proofpoint-GUID: H5gV-4cmryqEJruIJn0VFKzssbbBH8la Content-Transfer-Encoding: 7bit X-Proofpoint-UnRewURL: 0 URL was un-rewritten MIME-Version: 1.0 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.267,Aquarius:18.0.980,Hydra:6.0.601,FMLib:17.11.176.26 definitions=2023-09-18_08,2023-09-18_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 suspectscore=0 mlxlogscore=999 malwarescore=0 bulkscore=0 priorityscore=1501 adultscore=0 spamscore=0 phishscore=0 mlxscore=0 impostorscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2308100000 definitions=main-2309180155 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/18/23 5:52 PM, Valentin Schneider wrote: > 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. ok. > > In a pinch it also helps to know if CONFIG_ENERGY_MODEL was selected, > though that's obvious enough with CONFIG_SCHED_DEBUG=y. > ok. This would be simpler to implement as well. Removing it would have few tricky corner case scenarios as pierre has indicated. Should be able to send out v4 sometime soon. I am on a holiday till Sep 19. Pierre and Phil, thanks for the suggestions to commit message. I will incorporate the suggestions.