linux-kernel - Re: [PATCH 15/19] sched/fair: Respect LLC preference in task migration and detach

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Message-ID: <5cdf379c-b663-424d-8505-d91046e63c20@amd.com>
Date: Tue, 28 Oct 2025 11:32:30 +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: 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 15/19] sched/fair: Respect LLC preference in task
 migration and detach

Hello Tim,

On 10/11/2025 11:54 PM, Tim Chen wrote:
> @@ -9969,6 +9969,12 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>  	if (env->flags & LBF_ACTIVE_LB)
>  		return 1;
>  
> +#ifdef CONFIG_SCHED_CACHE
> +	if (sched_cache_enabled() &&
> +	    can_migrate_llc_task(env->src_cpu, env->dst_cpu, p) == mig_forbid)
> +		return 0;
> +#endif
> +
>  	degrades = migrate_degrades_locality(p, env);
>  	if (!degrades)
>  		hot = task_hot(p, env);

Should we care for task_hot() w.r.t. migration cost if a task is being
moved to a preferred LLC?

Also, should we leave out tasks under core scheduling from the llc
aware lb? Even discount them when calculating "mm->nr_running_avg"?

> @@ -10227,6 +10233,20 @@ static int detach_tasks(struct lb_env *env)
>  		if (env->imbalance <= 0)
>  			break;
>  
> +#ifdef CONFIG_SCHED_CACHE
> +		/*
> +		 * Don't detach more tasks if the remaining tasks want
> +		 * to stay. We know the remaining tasks all prefer the
> +		 * current LLC, because after order_tasks_by_llc(), the
> +		 * tasks that prefer the current LLC are at the tail of
> +		 * the list. The inhibition of detachment is to avoid too
> +		 * many tasks being migrated out of the preferred LLC.
> +		 */
> +		if (sched_cache_enabled() && detached && p->preferred_llc != -1 &&
> +		    llc_id(env->src_cpu) == p->preferred_llc)
> +			break;

In all cases? Should we check can_migrate_llc() wrt to util migrated and
then make a call if we should move the preferred LLC tasks or not?

Perhaps disallow it the first time if "nr_balance_failed" is 0 but
subsequent failed attempts should perhaps explore breaking the preferred
llc restriction if there is an imbalance and we are under
"mig_unrestricted" conditions.

> +#endif
> +
>  		continue;
>  next:
>  		if (p->sched_task_hot)

-- 
Thanks and Regards,
Prateek