Re: [PATCH v3 13/21] sched/cache: Handle moving single tasks to/from their preferred LLC

[Tim Chen]

On Fri, 2026-02-20 at 14:53 +0100, Peter Zijlstra wrote:
> On Tue, Feb 10, 2026 at 02:18:53PM -0800, Tim Chen wrote:
> > In the generic load balance(non-cache-aware-load-balance),
> > if the busiest runqueue has only one task, active balancing may be
> > invoked to move it. However, this migration might break LLC locality.
> 
> I'm thinking this wants more explanation; yes it might break locality,
> but why is that bad?

That's to prevent regular load balance from migrating a task that
prefers the current LLC, but the load level and imbalance doesn't warrant
breaking LLC preference per can_migrate_llc() policy.

Okay, will add more comments.

Tim

> 
> This way you're inhibiting a full LLC from being able to power down.
> 
> > +/*
> > + * Check if active load balance breaks LLC locality in
> > + * terms of cache aware load balance.
> > + */
> > +static inline bool
> > +alb_break_llc(struct lb_env *env)
> > +{
> > +	if (!sched_cache_enabled())
> > +		return false;
> > +
> > +	if (cpus_share_cache(env->src_cpu, env->dst_cpu))
> > +		return false;
> > +	/*
> > +	 * All tasks prefer to stay on their current CPU.
> > +	 * Do not pull a task from its preferred CPU if:
> > +	 * 1. It is the only task running there; OR
> > +	 * 2. Migrating it away from its preferred LLC would violate
> > +	 *    the cache-aware scheduling policy.
> > +	 */
> > +	if (env->src_rq->nr_pref_llc_running &&
> > +	    env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable) {
> > +		unsigned long util = 0;
> > +		struct task_struct *cur;
> > +
> > +		if (env->src_rq->nr_running <= 1)
> > +			return true;
> > +
> > +		/*
> > +		 * Reach here in load balance with
> > +		 * rcu_read_lock() protected.
> > +		 */
> 
> Not sure that comment helps much. rcu_dereference() itself will already
> cause complaints if the constraints are violated.

Okay, will remove this.

> 
> > +		cur = rcu_dereference(env->src_rq->curr);
> > +		if (cur)
> > +			util = task_util(cur);
> > +
> > +		if (can_migrate_llc(env->src_cpu, env->dst_cpu,
> > +				    util, false) == mig_forbid)
> > +			return true;
> > +	}
> > +
> > +	return false;
> > +}
> 
>