linux-kernel - Re: [PATCH 09/19] sched/fair: Count tasks prefering each LLC in a sched group

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Message-ID: <20251015122243.GV3289052@noisy.programming.kicks-ass.net>
Date: Wed, 15 Oct 2025 14:22:43 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: 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>, 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>, Chen Yu <yu.c.chen@...el.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 09/19] sched/fair: Count tasks prefering each LLC in a
 sched group

On Sat, Oct 11, 2025 at 11:24:46AM -0700, Tim Chen wrote:
> During LLC load balancing, tabulate the number of tasks on each runqueue
> that prefer a given destination LLC 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 runqueue. 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>
> ---
>  kernel/sched/fair.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b7a68fe7601b..cbd1e97bca4b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10399,6 +10399,9 @@ struct sg_lb_stats {
>  	unsigned int nr_numa_running;
>  	unsigned int nr_preferred_running;
>  #endif
> +#ifdef CONFIG_SCHED_CACHE
> +	unsigned int nr_pref_llc[NR_LLCS];
> +#endif
>  };

Hahahaha, no! We have this on-stack, this cannot be.