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Message-ID: <c9328e19-3b18-4ea3-a692-9cb02534e5c9@intel.com> Date: Sun, 22 Jun 2025 08:39:38 +0800 From: "Chen, Yu C" <yu.c.chen@...el.com> To: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>, Tim Chen <tim.c.chen@...ux.intel.com>, Peter Zijlstra <peterz@...radead.org>, "Ingo Molnar" <mingo@...hat.com>, K Prateek Nayak <kprateek.nayak@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com> CC: Juri Lelli <juri.lelli@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, Tim Chen <tim.c.chen@...el.com>, Vincent Guittot <vincent.guittot@...aro.org>, Libo Chen <libo.chen@...cle.com>, Abel Wu <wuyun.abel@...edance.com>, Hillf Danton <hdanton@...a.com>, Len Brown <len.brown@...el.com>, <linux-kernel@...r.kernel.org> Subject: Re: [RFC patch v3 00/20] Cache aware scheduling On 6/21/2025 3:25 AM, Madadi Vineeth Reddy wrote: > Hi Tim, > > On 18/06/25 23:57, Tim Chen wrote: >> This is the third revision of the cache aware scheduling patches, >> based on the original patch proposed by Peter[1]. >> >> The goal of the patch series is to aggregate tasks sharing data >> to the same cache domain, thereby reducing cache bouncing and >> cache misses, and improve data access efficiency. In the current >> implementation, threads within the same process are considered >> as entities that potentially share resources. >> >> In previous versions, aggregation of tasks were done in the >> wake up path, without making load balancing paths aware of >> LLC (Last-Level-Cache) preference. This led to the following >> problems: >> >> 1) Aggregation of tasks during wake up led to load imbalance >> between LLCs >> 2) Load balancing tried to even out the load between LLCs >> 3) Wake up tasks aggregation happened at a faster rate and >> load balancing moved tasks in opposite directions, leading >> to continuous and excessive task migrations and regressions >> in benchmarks like schbench. >> >> In this version, load balancing is made cache-aware. The main >> idea of cache-aware load balancing consists of two parts: >> >> 1) Identify tasks that prefer to run on their hottest LLC and >> move them there. >> 2) Prevent generic load balancing from moving a task out of >> its hottest LLC. >> >> By default, LLC task aggregation during wake-up is disabled. >> Conversely, cache-aware load balancing is enabled by default. >> For easier comparison, two scheduler features are introduced: >> SCHED_CACHE_WAKE and SCHED_CACHE_LB, which control cache-aware >> wake up and cache-aware load balancing, respectively. By default, >> NO_SCHED_CACHE_WAKE and SCHED_CACHE_LB are set, so tasks aggregation >> is only done on load balancing. > > Tested this patch series on a Power11 system with 28 cores and 224 CPUs. > LLC on this platform spans 4 threads. > > schbench: > baseline (sd%) baseline+cacheaware (sd%) %change > Lat 50.0th-worker-1 6.33 (24.12%) 6.00 (28.87%) 5.21% > Lat 90.0th-worker-1 7.67 ( 7.53%) 7.67 (32.83%) 0.00% > Lat 99.0th-worker-1 8.67 ( 6.66%) 9.33 (37.63%) -7.61% > Lat 99.9th-worker-1 21.33 (63.99%) 12.33 (28.47%) 42.19% > > Lat 50.0th-worker-2 4.33 (13.32%) 5.67 (10.19%) -30.95% > Lat 90.0th-worker-2 5.67 (20.38%) 7.67 ( 7.53%) -35.27% > Lat 99.0th-worker-2 7.33 ( 7.87%) 8.33 ( 6.93%) -13.64% > Lat 99.9th-worker-2 11.67 (24.74%) 10.33 (11.17%) 11.48% > > Lat 50.0th-worker-4 5.00 ( 0.00%) 7.00 ( 0.00%) -40.00% > Lat 90.0th-worker-4 7.00 ( 0.00%) 9.67 ( 5.97%) -38.14% > Lat 99.0th-worker-4 8.00 ( 0.00%) 11.33 (13.48%) -41.62% > Lat 99.9th-worker-4 10.33 ( 5.59%) 14.00 ( 7.14%) -35.53% > > Lat 50.0th-worker-8 4.33 (13.32%) 5.67 (10.19%) -30.95% > Lat 90.0th-worker-8 6.33 (18.23%) 8.67 ( 6.66%) -36.99% > Lat 99.0th-worker-8 7.67 ( 7.53%) 10.33 ( 5.59%) -34.69% > Lat 99.9th-worker-8 10.00 (10.00%) 12.33 ( 4.68%) -23.30% > > Lat 50.0th-worker-16 4.00 ( 0.00%) 5.00 ( 0.00%) -25.00% > Lat 90.0th-worker-16 6.33 ( 9.12%) 7.67 ( 7.53%) -21.21% > Lat 99.0th-worker-16 8.00 ( 0.00%) 10.33 ( 5.59%) -29.13% > Lat 99.9th-worker-16 12.00 ( 8.33%) 13.33 ( 4.33%) -11.08% > > Lat 50.0th-worker-32 5.00 ( 0.00%) 5.33 (10.83%) -6.60% > Lat 90.0th-worker-32 7.00 ( 0.00%) 8.67 (17.63%) -23.86% > Lat 99.0th-worker-32 10.67 (14.32%) 12.67 ( 4.56%) -18.75% > Lat 99.9th-worker-32 14.67 ( 3.94%) 19.00 (13.93%) -29.49% > > Lat 50.0th-worker-64 5.33 (10.83%) 6.67 ( 8.66%) -25.14% > Lat 90.0th-worker-64 10.00 (17.32%) 14.33 ( 4.03%) -43.30% > Lat 99.0th-worker-64 14.00 ( 7.14%) 16.67 ( 3.46%) -19.07% > Lat 99.9th-worker-64 55.00 (56.69%) 47.00 (61.92%) 14.55% > > Lat 50.0th-worker-128 8.00 ( 0.00%) 8.67 (13.32%) -8.38% > Lat 90.0th-worker-128 13.33 ( 4.33%) 14.33 ( 8.06%) -7.50% > Lat 99.0th-worker-128 16.00 ( 0.00%) 20.00 ( 8.66%) -25.00% > Lat 99.9th-worker-128 2258.33 (83.80%) 2974.67 (21.82%) -31.72% > > Lat 50.0th-worker-256 47.67 ( 2.42%) 45.33 ( 3.37%) 4.91% > Lat 90.0th-worker-256 3470.67 ( 1.88%) 3558.67 ( 0.47%) -2.54% > Lat 99.0th-worker-256 9040.00 ( 2.76%) 9050.67 ( 0.41%) -0.12% > Lat 99.9th-worker-256 13824.00 (20.07%) 13104.00 ( 6.84%) 5.21% > > The above data shows mostly regression both in the lesser and > higher load cases. > > > Hackbench pipe: > > Pairs Baseline Avg (s) (Std%) Patched Avg (s) (Std%) % Change > 2 2.987 (1.19%) 2.414 (17.99%) 24.06% > 4 7.702 (12.53%) 7.228 (18.37%) 6.16% > 8 14.141 (1.32%) 13.109 (1.46%) 7.29% > 15 27.571 (6.53%) 29.460 (8.71%) -6.84% > 30 65.118 (4.49%) 61.352 (4.00%) 5.78% > 45 105.086 (9.75%) 97.970 (4.26%) 6.77% > 60 149.221 (6.91%) 154.176 (4.17%) -3.32% > 75 199.278 (1.21%) 198.680 (1.37%) 0.30% > > A lot of run to run variation is seen in hackbench runs. So hard to tell > on the performance but looks better than schbench. May I know if the cpu frequency was set at a fixed level and deep cpu idle states were disabled(I assume on power system it is called stop states?) > > In Power 10 and Power 11, The LLC size is relatively smaller (4 CPUs) > when compared to platforms like sapphire rapids and Milan. Didn't go > through this series yet. Will go through and try to understand why > schbench is not happy on Power systems. > > Meanwhile, Wanted to know your thoughts on how does smaller LLC > size get impacted with this patch? > task aggregation on smaller LLC domain(both in terms of the number of CPUs and the size of LLC) might bring cache contention and hurt performance IMO. May I know what is the cache size on your system: lscpu | grep "L3 cache" May I know if you tested it with: echo NO_SCHED_CACHE > /sys/kernel/debug/sched/features echo NO_SCHED_CACHE_WAKE > /sys/kernel/debug/sched/features echo NO_SCHED_CACHE_LB > /sys/kernel/debug/sched/features vs echo SCHED_CACHE > /sys/kernel/debug/sched/features echo NO_SCHED_CACHE_WAKE > /sys/kernel/debug/sched/features echo SCHED_CACHE_LB > /sys/kernel/debug/sched/features And could you help check if setting /sys/kernel/debug/sched/llc_aggr_cap from 50 to some smaller values(25, etc) would help? thanks, Chenyu > Thanks, > Madadi Vineeth Reddy > > >> >> With above default settings, task migrations occur less frequently >> and no longer happen in the latency-sensitive wake-up path. >> > > [..snip..] > >> >> Chen Yu (3): >> sched: Several fixes for cache aware scheduling >> sched: Avoid task migration within its preferred LLC >> sched: Save the per LLC utilization for better cache aware scheduling >> >> K Prateek Nayak (1): >> sched: Avoid calculating the cpumask if the system is overloaded >> >> Peter Zijlstra (1): >> sched: Cache aware load-balancing >> >> Tim Chen (15): >> sched: Add hysteresis to switch a task's preferred LLC >> sched: Add helper function to decide whether to allow cache aware >> scheduling >> sched: Set up LLC indexing >> sched: Introduce task preferred LLC field >> sched: Calculate the number of tasks that have LLC preference on a >> runqueue >> sched: Introduce per runqueue task LLC preference counter >> sched: Calculate the total number of preferred LLC tasks during load >> balance >> sched: Tag the sched group as llc_balance if it has tasks prefer other >> LLC >> sched: Introduce update_llc_busiest() to deal with groups having >> preferred LLC tasks >> sched: Introduce a new migration_type to track the preferred LLC load >> balance >> sched: Consider LLC locality for active balance >> sched: Consider LLC preference when picking tasks from busiest queue >> sched: Do not migrate task if it is moving out of its preferred LLC >> sched: Introduce SCHED_CACHE_LB to control cache aware load balance >> sched: Introduce SCHED_CACHE_WAKE to control LLC aggregation on wake >> up >> >> include/linux/mm_types.h | 44 ++ >> include/linux/sched.h | 8 + >> include/linux/sched/topology.h | 3 + >> init/Kconfig | 4 + >> init/init_task.c | 3 + >> kernel/fork.c | 5 + >> kernel/sched/core.c | 25 +- >> kernel/sched/debug.c | 4 + >> kernel/sched/fair.c | 859 ++++++++++++++++++++++++++++++++- >> kernel/sched/features.h | 3 + >> kernel/sched/sched.h | 23 + >> kernel/sched/topology.c | 29 ++ >> 12 files changed, 982 insertions(+), 28 deletions(-) >> >
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