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

[Chen, Yu C]

On 9/29/2025 10:09 PM, Peter Zijlstra wrote:
> 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?

Ah right, will do it.

> > +	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?

OK, will write to it directly.

thanks,
Chenyu>> +}