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Mon, 27 Oct 2025 12:56:49 +0000 Message-ID: Date: Mon, 27 Oct 2025 20:56:29 +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: K Prateek Nayak 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 , , 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? I did a check on my VM: root@ubuntu:/sys/devices/system/cpu# lscpu CPU(s): 32 On-line CPU(s) list: 0-7 root@ubuntu:/sys/devices/system/cpu# echo 1 > cpu31/online Tracing ... Hit Ctrl-C to end. ^C @build_sched_domains[ build_sched_domains+5 partition_sched_domains+613 cpuset_update_active_cpus+838 sched_cpu_activate+272 cpuhp_invoke_callback+340 cpuhp_thread_fun+139 smpboot_thread_fn+238 kthread+249 ret_from_fork+193 ret_from_fork_asm+26 ]: 1 thanks, Chenyu