Re: [PATCH v8 13/15] futex: Resize local futex hash table based on number of threads.

[Peter Zijlstra]

On Mon, Feb 03, 2025 at 02:59:33PM +0100, Sebastian Andrzej Siewior wrote:
> Automatically size the local hash based on the number of threads. The
> logic tries to allocate between 16 and futex_hashsize (the default for
> the system wide hash bucket) and uses 4 * number-of-threads.
> On CONFIG_BASE_SMALL configs the suggested size is always 2.

> +	scoped_guard(rcu) {
> +		threads = get_nr_threads(current);

How about something like:

		threads = min(get_nr_threads(), num_online_cpus())

?

> +		hb_p = rcu_dereference(current->mm->futex_phash);
> +		if (hb_p)
> +			current_buckets = hb_p->hash_mask + 1;
> +	}
> +
> +	if (IS_ENABLED(CONFIG_BASE_SMALL)) {
> +		buckets = 2;
> +

Or... you just disable the local thing entirely for BASE_SMALL and have
it fall back to the global hash.

> +	} else {
> +		/*
> +		 * The default allocation will remain within
> +		 *   16 <= threads * 4 <= global hash size
> +		 */
> +		buckets = roundup_pow_of_two(4 * threads);
> +		buckets = max(buckets, 16);
> +		buckets = min(buckets, futex_hashsize);
> +	}
> +	if (current_buckets >= buckets)
> +		return 0;
> +
> +	return futex_hash_allocate(buckets);
>  }
>  
>  static int futex_hash_get_slots(void)
> -- 
> 2.47.2
>