Re: [PATCH v2 07/23] sched/cache: Introduce per runqueue task LLC preference counter

[Peter Zijlstra]

On Wed, Dec 03, 2025 at 03:07:26PM -0800, Tim Chen wrote:

> +static int resize_llc_pref(void)
> +{
> +	unsigned int *__percpu *tmp_llc_pref;
> +	int i, ret = 0;
> +
> +	if (new_max_llcs <= max_llcs)
> +		return 0;
> +
> +	/*
> +	 * Allocate temp percpu pointer for old llc_pref,
> +	 * which will be released after switching to the
> +	 * new buffer.
> +	 */
> +	tmp_llc_pref = alloc_percpu_noprof(unsigned int *);
> +	if (!tmp_llc_pref)
> +		return -ENOMEM;
> +
> +	for_each_present_cpu(i)
> +		*per_cpu_ptr(tmp_llc_pref, i) = NULL;
> +
> +	/*
> +	 * Resize the per rq nr_pref_llc buffer and
> +	 * switch to this new buffer.
> +	 */
> +	for_each_present_cpu(i) {
> +		struct rq_flags rf;
> +		unsigned int *new;
> +		struct rq *rq;
> +
> +		rq = cpu_rq(i);
> +		new = alloc_new_pref_llcs(rq->nr_pref_llc, per_cpu_ptr(tmp_llc_pref, i));
> +		if (!new) {
> +			ret = -ENOMEM;
> +
> +			goto release_old;
> +		}
> +
> +		/*
> +		 * Locking rq ensures that rq->nr_pref_llc values
> +		 * don't change with new task enqueue/dequeue
> +		 * when we repopulate the newly enlarged array.
> +		 */
> +		rq_lock_irqsave(rq, &rf);
> +		populate_new_pref_llcs(rq->nr_pref_llc, new);
> +		rq->nr_pref_llc = new;
> +		rq_unlock_irqrestore(rq, &rf);
> +	}
> +
> +release_old:
> +	/*
> +	 * Load balance is done under rcu_lock.
> +	 * Wait for load balance before and during resizing to
> +	 * be done. They may refer to old nr_pref_llc[]
> +	 * that hasn't been resized.
> +	 */
> +	synchronize_rcu();
> +	for_each_present_cpu(i)
> +		kfree(*per_cpu_ptr(tmp_llc_pref, i));
> +
> +	free_percpu(tmp_llc_pref);
> +
> +	/* succeed and update */
> +	if (!ret)
> +		max_llcs = new_max_llcs;
> +
> +	return ret;
> +}

> @@ -2674,6 +2787,8 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  	if (has_cluster)
>  		static_branch_inc_cpuslocked(&sched_cluster_active);
>  
> +	resize_llc_pref();
> +
>  	if (rq && sched_debug_verbose)
>  		pr_info("root domain span: %*pbl\n", cpumask_pr_args(cpu_map));

I suspect people will hate on you for that synchronize_rcu() in there.

Specifically, we do build_sched_domain() for every CPU brought online,
this means booting 512 CPUs now includes 512 sync_rcu()s.

Worse, IIRC sync_rcu() is O(n) (or worse -- could be n*ln(n)) in number
of CPUs, so the total thing will be O(n^2) (or worse) for bringing CPUs
online.