linux-kernel - Re: [RFC PATCH v4 06/28] sched: Save the per LLC utilization for better cache aware scheduling

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Message-ID: <20250929140920.GN3419281@noisy.programming.kicks-ass.net>
Date: Mon, 29 Sep 2025 16:09:20 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Chen Yu <yu.c.chen@...el.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>,
	Libo Chen <libo.chen@...cle.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>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aubrey Li <aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>,
	Chen Yu <yu.chen.surf@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 06/28] sched: Save the per LLC utilization for
 better cache aware scheduling

On Sat, Aug 09, 2025 at 01:02:54PM +0800, Chen Yu wrote:

> +#ifdef CONFIG_SCHED_CACHE
> +/*
> + * Save this sched group's statistic for later use:
> + * The task wakeup and load balance can make better
> + * decision based on these statistics.
> + */
> +static void update_sg_if_llc(struct lb_env *env, struct sg_lb_stats *sgs,
> +			     struct sched_group *group)
> +{
> +	/* Find the sched domain that spans 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 the sched domain that spans 1 LLC */
> +	if (!sd || !(sd->flags & SD_SHARE_LLC) ||
> +	    !sd->parent || (sd->parent->flags & SD_SHARE_LLC))
> +		return;

Did you want to write:

	if (sd != per_cpu(sd_llc))
		return;

Or something?

> +	sd_share = rcu_dereference(per_cpu(sd_llc_shared,
> +				  cpumask_first(sched_group_span(group))));
> +	if (!sd_share)
> +		return;
> +
> +	if (likely(READ_ONCE(sd_share->util_avg) != sgs->group_util))
> +		WRITE_ONCE(sd_share->util_avg, sgs->group_util);

If you expect it to be different, does that whole load and compare still
matter?

> +}