[PATCH v5 8/8] futex: Use runtime constants for __futex_hash() hot path

[Sebastian Andrzej Siewior]

On 2026-06-30 04:55:31 [+0000], K Prateek Nayak wrote:
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -395,13 +391,13 @@ __futex_hash(union futex_key *key, struct futex_private_hash *fph, struct futex_
>  		 * 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;
>  		if (!node_possible(node)) {
>  			node = find_next_bit_wrap(node_possible_map.bits, nr_node_ids, node);
>  		}

I replaced this with:

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 79e770d4d166..30d8622958d2 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -382,6 +382,7 @@ __futex_hash(union futex_key *key, struct futex_private_hash *fph, struct futex_
 		      key->both.offset);
 
 	if (node == FUTEX_NO_NODE) {
+		u32 node_limit = nr_node_ids;
 		/*
 		 * In case of !FLAGS_NUMA, use some unused hash bits to pick a
 		 * node -- this ensures regular futexes are interleaved across
@@ -391,9 +392,9 @@ __futex_hash(union futex_key *key, struct futex_private_hash *fph, struct futex_
 		 * NOTE: this isn't perfectly uniform, but it is fast and
 		 * handles sparse node masks.
 		 */
-		node = runtime_const_shift_right_32(hash, __futex_shift) % nr_node_ids;
-		if (!node_possible(node)) {
-			node = find_next_bit_wrap(node_possible_map.bits, nr_node_ids, node);
+		node = reciprocal_scale(hash, node_limit);
+		if (!node_possible(node)) {
+			node = find_next_bit_wrap(node_possible_map.bits, node_limit, node);
 		}
 	}
 
I don't think it is worse, I hardly see a change perf wise. Sometimes
op/s is reported almost unchanged, sometimes it improves a bit.

What it does it reads nr_node_ids only once (which has no effect here
because I have no sparse node) and it replaces the shift + divl with
imulq + shift.

perf was pointing me to the divl but now it points to the imulq.
¯\_(ツ)_/¯

But having that div gone, can't be bad, can it?

Sebastian