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: <7ff17661-05fe-4137-b7e1-c799fe318c9f@amd.com>
Date: Mon, 27 Oct 2025 11:12:07 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Tim Chen <tim.c.chen@...ux.intel.com>, Peter Zijlstra
	<peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, "Gautham R . Shenoy"
	<gautham.shenoy@....com>
CC: Chen Yu <yu.c.chen@...el.com>, 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>, Libo Chen <libo.chen@...cle.com>, Adam Li
	<adamli@...amperecomputing.com>, Tim Chen <tim.c.chen@...el.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache
 aware only for multi LLCs

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?

> +				}
>  				imb = max(1U, imb);
>  				sd->imb_numa_nr = imb;
>  
> @@ -2581,6 +2584,13 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  	if (has_cluster)
>  		static_branch_inc_cpuslocked(&sched_cluster_active);
>  
> +#ifdef CONFIG_SCHED_CACHE
> +	if (has_multi_llcs) {
> +		static_branch_enable_cpuslocked(&sched_cache_allowed);
> +		pr_info("Cache aware load balance enabled.\n");
> +	}
> +#endif
> +
>  	if (rq && sched_debug_verbose)
>  		pr_info("root domain span: %*pbl\n", cpumask_pr_args(cpu_map));
>  

-- 
Thanks and Regards,
Prateek