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Wed, 29 Oct 2025 12:36:55 +0000 Message-ID: <1540b472-b5b4-4e10-af88-816cc256931b@intel.com> Date: Wed, 29 Oct 2025 20:36:38 +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: Tim Chen CC: Vincent Guittot , Juri Lelli , Dietmar Eggemann , "Steven Rostedt" , Ben Segall , Mel Gorman , Valentin Schneider , "Madadi Vineeth Reddy" , Hillf Danton , "Shrikanth Hegde" , Jianyong Wu , "Yangyu Chen" , Tingyin Duan , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Tim Chen , , Peter Zijlstra , "Gautham R . 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>>>> - if (nr_llcs == 1) >>>> + if (nr_llcs == 1) { >>>> imb = sd->span_weight >> 3; >>>> - else >>>> + } else { >>>> imb = nr_llcs; >>>> + has_multi_llcs = true; >>> >>> One caution: this will not hold if all the CPUs aren't online during boot. >>> One case I can think of is when the kernel is booted with "maxcpus" cmdline >>> and CPUs are hotplugged later. >>> >>> Unfortunately, I don't think we even have the raw topology data from the >>> arch/ side under such scenario to accurately make a call if the system >>> contains single or multiple LLC :( >>> >>> I'm not sure if it is feasible but assuming the task_work() cannot run if >>> &sched_cache_allowed is false, can the fist instance of the task work for >>> sched_cache do the necessary setup? >>> >> >> build_sched_domains() might get invoked to rebuild the corresponding sched >> domains during CPU hotplug via cpuset subsystem. So if the CPU gets online >> after bootup, we still have the chance to detect multiple LLCs I suppose? > > The case Pratek brought up of adding CPUs and enabling SCHED_CACHE > should be covered. > > The trickier case is if we disable SCHED_CACHE when CPUs are > offlined and multi_cpus becomes false. We'll need to clear out rq->nr_pref_llcs > data and tasks' preferred LLC would need to be cleared. Or else the accounting > could be skewed we bring CPU online later and again re-enable SCHED_CACHE. To safely keep the data consistent, do we need to add hook when SCHED_CACHE is disabled at runtime, to clear all the stale data? In that way, another race condition between SCHED_CACHE enabling vs EQ/DQ might also be covered. thanks, Chenyu > So far we haven't done that when we disable SCHED_CACHE from an enabled state. >