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

[Chen, Yu C]

On 10/29/2025 5:51 PM, Aaron Lu wrote:
> On Mon, Oct 27, 2025 at 10:00:52AM +0800, Chen, Yu C wrote:
> > Hi Aaron,
> >
> > On 10/24/2025 5:32 PM, Aaron Lu wrote:
> > > Hi Tim,
> > >
> > > On Sat, Oct 11, 2025 at 11:24:47AM -0700, Tim Chen wrote:
> > > > @@ -10849,11 +10849,45 @@ static void record_sg_llc_stats(struct lb_env *env,
> > > >    	if (unlikely(READ_ONCE(sd_share->capacity) != sgs->group_capacity))
> > > >    		WRITE_ONCE(sd_share->capacity, sgs->group_capacity);
> > > >    }
> > > > +
> > > > +/*
> > > > + * Do LLC balance on sched group that contains LLC, and have tasks preferring
> > > > + * to run on LLC in idle dst_cpu.
> > > > + */
> > > > +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 &&
> > > > +	    can_migrate_llc(env->src_cpu, env->dst_cpu, 0, true) == mig_llc)
> > >
> > > llc_balance() is called from update_sg_lb_stats() and at that time,
> > > env->src_cpu is not determined yet so should not be used here?
> >
> > You are right, I think we should check the candidate group's first
> > CPU rather than the env->src_cpu. Will fix it in the next version.
>
> Looks like can_migrate_llc() doesn't care an exact cpu but any cpu in the
> same LLC should do, so either the candidate group's first cpu or any
> other cpus in that group should make no difference.

Yes, actually the cache aware is based on LLC rather than CPU. It was
a historic reason that the original proposal was based on CPU in the
wakeup path.

> It might be more intuitive to prototype can_migrate_llc() with sd_shared
> as param than using cpu, just a thought.

I understand your concern,  will think about this.

thanks,
Chenyu