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Wed, 24 Dec 2025 07:51:28 +0000 Message-ID: Date: Wed, 24 Dec 2025 15:51:16 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 20/23] sched/cache: Add user control to adjust the parameters of cache-aware scheduling To: Yangyu Chen , Tim Chen CC: Peter Zijlstra , 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 , Shrikanth Hegde , Jianyong Wu , Tingyin Duan , Vern Hao , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Aaron Lu , Tim Chen , , Qais Yousef References: Content-Language: en-US From: "Chen, Yu C" In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: TPYP295CA0033.TWNP295.PROD.OUTLOOK.COM (2603:1096:7d0:7::17) To PH7PR11MB6005.namprd11.prod.outlook.com (2603:10b6:510:1e0::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: PH7PR11MB6005:EE_|MN6PR11MB8172:EE_ X-MS-Office365-Filtering-Correlation-Id: 353a0c94-7e59-4cb6-1813-08de42c13e9d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|366016|1800799024|376014|7053199007; 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If RSS exceeds the LLC >>>> size, aggregation is skipped. Some workloads with large RSS but small >>>> actual memory footprints may still benefit from aggregation. Since >>>> the kernel cannot efficiently track per-task cache usage (resctrl is >>>> user-space only), userspace can provide a more accurate hint. >>>> >>>> Introduce /sys/kernel/debug/sched/llc_aggr_tolerance to let >>>> users control how strictly RSS limits aggregation. Values range from >>>> 0 to 100: >>>> >>>> - 0: Cache-aware scheduling is disabled. >>>> - 1: Strict; tasks with RSS larger than LLC size are skipped. >>>> - 100: Aggressive; tasks are aggregated regardless of RSS. >>>> >>> >>> Hi Chen Yu and Tim Chen, >>> >>> Maybe we should have something like prctl(PR_LLC_AGGR_TOLERANCE, 100). >>> >>> I have tested this version of the patch on my EPYC Milan 7V13 (7763 variant) physical machine, with 32M LLC for each 8-core CCX. I found that I need to tune "llc_aggr_tolerance" to 100, else I can't get cache-aware scheduling to work on Verilated [1] XiangShan [2] running the chacha20 [3] as I mentioned before [4]. >>> >> >> In addition, I have investigated why this happens. And finally I >> realized that's because that workload observed 35596 kB RssAnon on >> my EPYC Milan Machine, slightly exceeding the LLC size (32M). I >> have tested it on an EPYC Genoa cloud server with the correct core >> / cache hierarchy in ACPI table, that shows 31700 kB RssAnon, thus >> fitting in LLC. I have no idea why my result shows higher RssAnon, >> since they both run Debian Trixie with the exact same kernel and >> same executable. But it reminds me we should have a userspace API >> for that. >> > > In addition, during profiling the verilator, I found that if scheduled > to SMTs, it will result in poor performance. Thus, I think we should > separate the control for rss size with the SMT scale. > Thanks for the investigation. Could you elaborate a little more about scheduled to SMTs? Do you mean, if every CPU(SMT) in the LLC has 1 running task, then the performance is impacted? I thought we have exceed_llc_nr() to check the smt to avoid this? > It's notable that rss size is not the actual memory footprint. It > would be better if we could measure the l2_miss event or l3_miss > event to measure the l3 hit rate. Just for future work. > Yes, in user space, we can collect PMUs events/memory bandwidth via resctrl to decide whether to set task attributes. > I'm willing to provide a patch for such a prctl. But I'm busy these > days, maybe I can have the time to do that after one week. > Sure. We haven't yet decided which interface we can leverage. Also, Qais is working on QOS interface[1] - maybe we can build on his work. [1] https://lore.kernel.org/all/20240820163512.1096301-11-qyousef@layalina.io/ thanks, Chenyu