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

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Message-ID: <20251015101519.GN3289052@noisy.programming.kicks-ass.net>
Date: Wed, 15 Oct 2025 12:15:19 +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>,
	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

On Sat, Oct 11, 2025 at 11:24:39AM -0700, Tim Chen wrote:
> +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;
> +	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))));

Isn't this sd->shared ? Or did I loose the plot somewhere?

> +	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);
> +}