Re: [PATCH v8 03/15] futex: Add basic infrastructure for local task local hash.

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 03, 2025 at 02:59:23PM +0100, Sebastian Andrzej Siewior wrote:

>  struct futex_hash_bucket *futex_hash(union futex_key *key)
>  {
> -	u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
> -			  key->both.offset);
> +	struct futex_hash_bucket *fhb;
> +	u32 hash;
>  
> +	fhb = current->mm->futex_hash_bucket;
> +	if (fhb && futex_key_is_private(key)) {
> +		u32 hash_mask = current->mm->futex_hash_mask;
> +
> +		hash = jhash2((u32 *)key,
> +			      offsetof(typeof(*key), both.offset) / 4,
> +			      key->both.offset);
> +		return &fhb[hash & hash_mask];
> +	}
> +	hash = jhash2((u32 *)key,
> +		      offsetof(typeof(*key), both.offset) / 4,
> +		      key->both.offset);
>  	return &futex_queues[hash & (futex_hashsize - 1)];
>  }

Having read this later patch, I would still prefer this to look like:

struct futex_hash_bucker *futex_hash(union futex_key *key)
{
	struct futex_hash_bucket *fhb = current->mm->futex_hash_bucket;
	u32 hash;

	if (futex_key_is_private) {
		// shorter jhash invocation;
	} else {
		hash = jhash2((u32 *)key,
				offsetof(typeof(*key), both.offset) / 4,
				key->both.offset);
	}

	if (fhb)
		return fhb[hash & current->mm->futex_hash_mask];

	return &futex_queues[hash & (futex_hashsize - 1)];
}

Because the shorter hash invocation only cares about being private, not
about having fhb.

Notably, I'm also still thikning about all that pending FUTEX2_NUMA
stuff.