Re: [RFC PATCH] futex: Dynamically allocate futex_queues depending on nr_node_ids

[Sebastian Andrzej Siewior]

On 2026-02-27 17:18:41 [+0100], Peter Zijlstra wrote:
> Ooh, I just remebered, I've always wanted to apply Linus' runtime-const
> stuff to the futex thing.

I wasn't aware we have this, I did want to do/ have something like this.

Need to look at this in detail but…

> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -439,14 +442,14 @@ __futex_hash(union futex_key *key, struct futex_private_hash *fph)
>  		 * NOTE: this isn't perfectly uniform, but it is fast and
>  		 * handles sparse node masks.
>  		 */
> -		node = (hash >> futex_hashshift) % nr_node_ids;
> +		node = runtime_const_shift_right_32(hash, __futex_shift) % nr_node_ids;

getting rid of that div here would be the next step. If we could round
it up to the next pow2 then we could avoid it. But I remember it did not
show up in bench as much as I was thinking it would.

>  		if (!node_possible(node)) {

Sebastian