linux-kernel - Re: [PATCH v2 02/23] sched/cache: Record per-LLC utilization to guide cache-aware scheduling decisions

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Message-ID: <20251209112134.GK3707891@noisy.programming.kicks-ass.net>
Date: Tue, 9 Dec 2025 12:21:34 +0100
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>,
	Chen Yu <yu.c.chen@...el.com>, 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>, Vern Hao <haoxing990@...il.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>,
	Adam Li <adamli@...amperecomputing.com>,
	Aaron Lu <ziqianlu@...edance.com>, Tim Chen <tim.c.chen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/23] sched/cache: Record per-LLC utilization to
 guide cache-aware scheduling decisions

On Wed, Dec 03, 2025 at 03:07:21PM -0800, Tim Chen wrote:

> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index bbcfdf12aa6e..0ba4697d74ba 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -68,6 +68,10 @@ struct sched_domain_shared {
>  	atomic_t	nr_busy_cpus;
>  	int		has_idle_cores;
>  	int		nr_idle_scan;
> +#ifdef CONFIG_SCHED_CACHE
> +	unsigned long	util_avg;
> +	unsigned long	capacity ____cacheline_aligned_in_smp;

This cacheline annotation confuses me, see below.

> +#endif
>  };
>  
>  struct sched_domain {
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cb82f558dc5b..b9f336300f14 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9622,6 +9622,29 @@ static inline int task_is_ineligible_on_dst_cpu(struct task_struct *p, int dest_
>  	return 0;
>  }
>  
> +#ifdef CONFIG_SCHED_CACHE
> +/* Called from load balancing paths with rcu_read_lock held */
> +static __maybe_unused bool get_llc_stats(int cpu, unsigned long *util,
> +					 unsigned long *cap)
> +{
> +	struct sched_domain_shared *sd_share;
> +
> +	sd_share = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> +	if (!sd_share)
> +		return false;
> +
> +	*util = READ_ONCE(sd_share->util_avg);
> +	*cap = READ_ONCE(sd_share->capacity);

You placed capacity on a separate line, forcing the above to be 2
distinct lines. That seems... sub-optimal?

> +
> +	return true;
> +}
> +#else
> +static inline bool get_llc_stats(int cpu, unsigned long *util,
> +				 unsigned long *cap)
> +{
> +	return false;
> +}
> +#endif
>  /*
>   * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
>   */
> @@ -10592,6 +10615,51 @@ sched_reduced_capacity(struct rq *rq, struct sched_domain *sd)
>  	return check_cpu_capacity(rq, sd);
>  }
>  
> +#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)
> +{
> +	struct sched_domain_shared *sd_share;
> +
> +	if (!sched_cache_enabled() || env->idle == CPU_NEWLY_IDLE)
> +		return;
> +
> +	/* Only care about sched domain spanning multiple LLCs */
> +	if (env->sd->child != 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.
> +	 * Note: sd_share cannot be obtained via sd->child->shared, because
> +	 * it refers to the domain that covers the local group, while
> +	 * sd_share could represent any of the LLC group.
> +	 */
> +	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);

And same here.

> +}
> +#else
> +static inline void record_sg_llc_stats(struct lb_env *env, struct sg_lb_stats *sgs,
> +				       struct sched_group *group)
> +{
> +}
> +#endif