linux-kernel - Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache aware only for multi LLCs

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Message-ID: <ebc82974-7fe5-4e9a-8dae-7964cfdff471@intel.com>
Date: Mon, 27 Oct 2025 20:56:29 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: K Prateek Nayak <kprateek.nayak@....com>
CC: Vincent Guittot <vincent.guittot@...aro.org>, 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>, "Madadi Vineeth
 Reddy" <vineethr@...ux.ibm.com>, Hillf Danton <hdanton@...a.com>, "Shrikanth
 Hegde" <sshegde@...ux.ibm.com>, Jianyong Wu <jianyong.wu@...look.com>,
	"Yangyu Chen" <cyy@...self.name>, Tingyin Duan <tingyin.duan@...il.com>, Vern
 Hao <vernhao@...cent.com>, Len Brown <len.brown@...el.com>, Aubrey Li
	<aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>, Chen Yu
	<yu.chen.surf@...il.com>, Adam Li <adamli@...amperecomputing.com>, Tim Chen
	<tim.c.chen@...el.com>, <linux-kernel@...r.kernel.org>, Tim Chen
	<tim.c.chen@...ux.intel.com>, Peter Zijlstra <peterz@...radead.org>, "Gautham
 R . Shenoy" <gautham.shenoy@....com>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache
 aware only for multi LLCs

Hi Prateek,

On 10/27/2025 1:42 PM, K Prateek Nayak wrote:
> Hello Tim,
> 
> On 10/11/2025 11:54 PM, Tim Chen wrote:
>> @@ -2530,10 +2531,12 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>>   				 * between LLCs and memory channels.
>>   				 */
>>   				nr_llcs = sd->span_weight / child->span_weight;
>> -				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@...ntu:/sys/devices/system/cpu# lscpu
CPU(s):                      32
   On-line CPU(s) list:       0-7
root@...ntu:/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