Re: [PATCH 10/19] sched/fair: Prioritize tasks preferring destination LLC during balancing

[Chen, Yu C]

On 10/15/2025 11:10 PM, Peter Zijlstra wrote:
> On Sat, Oct 11, 2025 at 11:24:47AM -0700, Tim Chen wrote:
>
> > +static inline bool llc_balance(struct lb_env *env, struct sg_lb_stats *sgs,
> > +			       struct sched_group *group)
> > +{
> > +	struct sched_domain *child = env->sd->child;
> > +	int llc;
> > +
> > +	if (!sched_cache_enabled())
> > +		return false;
> > +
> > +	if (env->sd->flags & SD_SHARE_LLC)
> > +		return false;
> > +
> > +	/* only care about task migration among LLCs */
> > +	if (child && !(child->flags & SD_SHARE_LLC))
> > +		return false;
> > +
> > +	llc = llc_idx(env->dst_cpu);
> > +	if (sgs->nr_pref_llc[llc] > 0 &&
>
> Robot says llc can be -1 here, and it don't like doing out of bound
> array access.

Humm, there seems to be a race condition during bootup with 
build_sched_domains(),
where the per-cpu(sd_llc_idx) is reset to -1 at the beginning. We might
need to add a sanity check here.


thanks,
Chenyu
> > +	    can_migrate_llc(env->src_cpu, env->dst_cpu, 0, true) == mig_llc)
> > +		return true;
> > +
> > +	return false;
> > +}