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

[Peter Zijlstra]

On Wed, Dec 10, 2025 at 11:02:39PM +0900, Chen, Yu C wrote:
> 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?

But its introduced here and already read/written together. Is this not
premature optimization?