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Wed, 15 Oct 2025 17:01:21 +0000 Message-ID: <3d7b6b2c-7b48-4bd5-87bf-39f4c72fa741@intel.com> Date: Thu, 16 Oct 2025 01:01:05 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache aware only for multi LLCs To: Shrikanth Hegde CC: Ingo Molnar , K Prateek Nayak , "Gautham R . Shenoy" , Vincent Guittot , Juri Lelli , "Dietmar Eggemann" , Steven Rostedt , Ben Segall , Mel Gorman , "Valentin Schneider" , Madadi Vineeth Reddy , Hillf Danton , Jianyong Wu , Yangyu Chen , Tingyin Duan , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Tim Chen , , Peter Zijlstra , "Tim Chen" References: <20251015110450.GO3289052@noisy.programming.kicks-ass.net> <15b871f2-49b8-4b62-926d-31f93ad49f51@intel.com> <6007f50a-7abd-46c8-8934-f1b09df57479@linux.ibm.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <6007f50a-7abd-46c8-8934-f1b09df57479@linux.ibm.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: KL1PR01CA0073.apcprd01.prod.exchangelabs.com (2603:1096:820:2::13) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR11MB6020:EE_|CYXPR11MB8712:EE_ X-MS-Office365-Filtering-Correlation-Id: a87b1b5f-428c-4963-b29a-08de0c0c7714 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|1800799024; 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>>>>   __read_mostly unsigned int llc_imb_pct            = 20; >>>> +DEFINE_STATIC_KEY_FALSE(sched_cache_allowed); >>>> + >>>> +static inline bool sched_cache_enabled(void) >>>> +{ >>>> +    return sched_feat(SCHED_CACHE) && >>>> +        static_branch_likely(&sched_cache_allowed); >>>> +} >>> >>> Urgh; do we really need _2_ static keys stacked for this? I'm thinking >>> one should be well enough. >> >> SCHED_CACHE allows user space to turn on/off the feature at runtime, >> while sched_cache_allow is a hardware capability. This capability is > > isn't it possible use only static_branch_likely(&sched_cache_allowed) at > runtime? > > Enable that key only if FEAT is set. Disable when unset. > That way you could use only one static branch at runtime. > Oh, do you mean only using sched_cache_allowed in sched_cache_enabled()? I misunderstood that Peter suggested introducing only one key. I did not quite catch up, do you mean, we should monitor the switch of FEAT and modify sched_cache_allowed when needed, and the OS only queries the sched_cache_allowed at runtime? I'll take a deeper look tomorrow. Oh, do you mean only using sched_cache_allowed in sched_cache_enabled()? I misunderstood that Peter suggested introducing only one key. But I didn't quite catch up - do you mean we should monitor the switch of FEAT, modify sched_cache_allowed when needed, and that the OS only queries sched_cache_allowed at runtime? > Also, I am not sure if the FEATURE should be true by default. I know it > maybe unused but > IMO it should be true by default only when its proven there are no > regression. Yes we tried very hard to not bring regressions during the past. > One should be aware of their topology to enable it. >>   disabled if there are no multiple LLCs within one node. I’m not sure >> if using one key could support the above two scenarios. >> It is possible to have multiple NUMA nodes. One node may have multiple >> LLC while other > one may have only 1 LLC. what happens in that case? > In this case, it will be enabled, and the cache-aware load balancing will occur on that node with multiple LLCs. (Only a domain with SD_SHARE_LLC set, and whose parent domain does not have SD_SHARE_LLC set, will initiate the cache-aware load balancing.) thanks, Chenyu > I am yet to go through the series(hopefully this week). Maybe its > handled already.