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

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Message-ID: <20250203144101.GL7145@noisy.programming.kicks-ass.net>
Date: Mon, 3 Feb 2025 15:41:01 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org,
	André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v8 03/15] futex: Add basic infrastructure for local task
 local hash.

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.