linux-kernel - Re: [PATCH v2 11/14] futex: Implement FUTEX2

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Date:   Mon, 07 Aug 2023 23:11:04 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>, axboe@...nel.dk
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, dvhart@...radead.org, dave@...olabs.net,
        andrealmeid@...lia.com, Andrew Morton <akpm@...ux-foundation.org>,
        urezki@...il.com, hch@...radead.org, lstoakes@...il.com,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        malteskarupke@....de
Subject: Re: [PATCH  v2 11/14] futex: Implement FUTEX2_NUMA

On Mon, Aug 07 2023 at 14:18, Peter Zijlstra wrote:
>  /**
>   * futex_hash - Return the hash bucket in the global hash
>   * @key:	Pointer to the futex key for which the hash is calculated
> @@ -114,10 +137,29 @@ late_initcall(fail_futex_debugfs);
>   */
>  struct futex_hash_bucket *futex_hash(union futex_key *key)
>  {
> -	u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
> +	u32 hash = jhash2((u32 *)key,
> +			  offsetof(typeof(*key), both.offset) / sizeof(u32),
>  			  key->both.offset);
> +	int node = key->both.node;
> +
> +	if (node == -1) {

NUMA_NO_NODE please all over the place.

> +		/*
> +		 * In case of !FLAGS_NUMA, use some unused hash bits to pick a
> +		 * node -- this ensures regular futexes are interleaved across
> +		 * the nodes and avoids having to allocate multiple
> +		 * hash-tables.
> +		 *
> +		 * NOTE: this isn't perfectly uniform, but it is fast and
> +		 * handles sparse node masks.
> +		 */
> +		node = (hash >> futex_hashshift) % nr_node_ids;
> +		if (!node_possible(node)) {
> +			node = find_next_bit_wrap(node_possible_map.bits,
> +						  nr_node_ids, node);
> +		}

Smart.

>  
> +static inline unsigned int futex_size(unsigned int flags)
> +{
> +	return 1 << (flags & FLAGS_SIZE_MASK);
> +}
> +
>  static inline bool futex_flags_valid(unsigned int flags)

If you reorder these two functions in the patch which introduces them,
this diff gets readable :)

Aside of that this thing is really hard to review :)