linux-kernel - Re: [PATCH v8 06/15] futex: Move private hashing into its own function.

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Message-ID: <20250205075117.Qho6oGnQ@linutronix.de>
Date: Wed, 5 Feb 2025 08:51:17 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
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 06/15] futex: Move private hashing into its own
 function.

On 2025-02-04 10:34:02 [+0100], Peter Zijlstra wrote:
> > index f608cd6ccc032..fdfc3402278a1 100644
> > --- a/kernel/futex/core.c
> > +++ b/kernel/futex/core.c
> > @@ -131,14 +143,9 @@ struct futex_hash_bucket *futex_hash(union futex_key *key)
> >  	u32 hash;
> >  
> >  	fhb = current->mm->futex_hash_bucket;
> > -	if (fhb && futex_key_is_private(key)) {
> > -		u32 hash_mask = current->mm->futex_hash_mask;
> > +	if (fhb && futex_key_is_private(key))
> > +		return futex_hash_private(key, fhb, current->mm->futex_hash_mask);
> >  
> > -		hash = jhash2((void *)&key->private.address,
> > -			      sizeof(key->private.address) / 4,
> > -			      key->both.offset);
> > -		return &fhb[hash & hash_mask];
> > -	}
> >  	hash = jhash2((u32 *)key,
> >  		      offsetof(typeof(*key), both.offset) / 4,
> >  		      key->both.offset);
> 
> Humph, this is a lot of back and forth. Maybe just do it right once?

I pulled it in little steps and I replaced it step by step.
I stayed with the "smaller" jhash2 because it is less code. We might do
something like hash_long() which is even less. I had numbers at
	https://lore.kernel.org/all/20241101110810.R3AnEqdu@linutronix.de/

But I can pull it in as suggested.

Sebastian