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

[Chen, Yu C]

On 12/9/2025 8:21 PM, Peter Zijlstra wrote:
> 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?

The reason capacity was placed in a separate cache line
is that writes to capacity are not very frequent(cpu hotplug
should not happen too-frequently), while writes to util_avg
tend to be relatively frequent(changes frequently).
If capacity and util_avg were placed in the same cache line,
I’m thinking writes to util_avg might invalidate the entire
cache line. This could potentially cause cache misses when
capacity is read elsewhere, which might lead to false sharing?

thanks,
Chenyu

> > +
> > +	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