linux-kernel - Re: [PATCH 02/19] sched/fair: Record per-LLC utilization to guide cache-aware scheduling decisions

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Message-ID: <5f97134b-71b6-44fb-85fd-5cff6725cf9c@amd.com>
Date: Mon, 27 Oct 2025 10:31:58 +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 02/19] sched/fair: Record per-LLC utilization to guide
 cache-aware scheduling decisions

Hello Tim,

On 10/11/2025 11:54 PM, Tim Chen wrote:
> +#ifdef CONFIG_SCHED_CACHE
> +/*
> + * Record the statistics for this scheduler group for later
> + * use. These values guide load balancing on aggregating tasks
> + * to a LLC.
> + */
> +static void record_sg_llc_stats(struct lb_env *env,
> +				struct sg_lb_stats *sgs,
> +				struct sched_group *group)
> +{
> +	/*
> +	 * Find the child domain on env->dst_cpu. This domain
> +	 * is either the domain that spans this group(if the
> +	 * group is a local group), or the sibling domain of
> +	 * this group.
> +	 */
> +	struct sched_domain *sd = env->sd->child;

Was this intentionally done to limit the update to sg_llc_stats to the
load balancing period of "sd_llc->parent"?

Can't this be done with update_idle_cpu_scan()? I believe it is more
frequent, "sds->total_capacity" from caller gives you the equivalent of
"group_capacity", and "group_util" is already calculated as "sum_util".

Checking "sd_llc->parent" there should be sufficient to check if there
are multiple LLC domains or not. Thoughts?

> +	struct sched_domain_shared *sd_share;
> +
> +	if (!sched_feat(SCHED_CACHE) || env->idle == CPU_NEWLY_IDLE)
> +		return;
> +
> +	/* only care about sched domains spanning a LLC */
> +	if (sd != rcu_dereference(per_cpu(sd_llc, env->dst_cpu)))
> +		return;
> +
> +	/*
> +	 * At this point we know this group spans a LLC domain.
> +	 * Record the statistic of this group in its corresponding
> +	 * shared LLC domain.
> +	 */
> +	sd_share = rcu_dereference(per_cpu(sd_llc_shared,
> +					   cpumask_first(sched_group_span(group))));
> +	if (!sd_share)
> +		return;
> +
> +	if (READ_ONCE(sd_share->util_avg) != sgs->group_util)
> +		WRITE_ONCE(sd_share->util_avg, sgs->group_util);
> +
> +	if (unlikely(READ_ONCE(sd_share->capacity) != sgs->group_capacity))
> +		WRITE_ONCE(sd_share->capacity, sgs->group_capacity);
> +}

-- 
Thanks and Regards,
Prateek