linux-kernel - Re: [PATCH v2 09/23] sched/cache: Count tasks prefering destination LLC in a sched group

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Message-ID: <5613f6eb-7319-464e-9723-2d82ddaca0c9@intel.com>
Date: Wed, 10 Dec 2025 23:05:33 +0900
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, Tim Chen
	<tim.c.chen@...ux.intel.com>
CC: Ingo Molnar <mingo@...hat.com>, K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....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>, Vern Hao <haoxing990@...il.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>, Aaron Lu <ziqianlu@...edance.com>, Tim Chen
	<tim.c.chen@...el.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 09/23] sched/cache: Count tasks prefering destination
 LLC in a sched group

On 12/10/2025 9:52 PM, Peter Zijlstra wrote:
> On Wed, Dec 03, 2025 at 03:07:28PM -0800, Tim Chen wrote:
>> During LLC load balancing, tabulate the number of tasks on each runqueue
>> that prefer the LLC contains the env->dst_cpu in a sched group.
>>
>> For example, consider a system with 4 LLC sched groups (LLC0 to LLC3)
>> balancing towards LLC3. LLC0 has 3 tasks preferring LLC3, LLC1 has
>> 2, and LLC2 has 1. LLC0, having the most tasks preferring LLC3, is
>> selected as the busiest source to pick tasks from.
>>
>> Within a source LLC, the total number of tasks preferring a destination
>> LLC is computed by summing counts across all CPUs in that LLC. For
>> instance, if LLC0 has CPU0 with 2 tasks and CPU1 with 1 task preferring
>> LLC3, the total for LLC0 is 3.
>>
>> These statistics allow the load balancer to choose tasks from source
>> sched groups that best match their preferred LLCs.
>>
>> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
>> ---
>>
>> Notes:
>>      v1->v2:
>>          Convert nr_pref_llc array in sg_lb_stats to a single
>>          variable as only the dst LLC stat is needed.
>>          (K Prateek Nayak)
>>
>>   kernel/sched/fair.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index b0e87616e377..4d7803f69a74 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -10445,6 +10445,9 @@ struct sg_lb_stats {
>>   	unsigned int nr_numa_running;
>>   	unsigned int nr_preferred_running;
>>   #endif
>> +#ifdef CONFIG_SCHED_CACHE
>> +	unsigned int nr_pref_llc;
>> +#endif
> 
> At this point I have to note that rq->nr_pref_llc seems like a horrible
> misnomer, for it being an array, and not an actual number like the
> naming suggests.

In the v2 it seems that rq->nr_pref_llc is not an array anymore, it 
indicates
the number of tasks that want to be migrated to the env->dst_cpu 
(dst_llc), because
these tasks' preferred LLC are env->dst_cpu(dst_llc). Maybe renaming it to
rq->nr_pref_dst_llc?

thanks,
Chenyu