linux-kernel - Re: [PATCH v10 20/21] futex: Implement FUTEX2

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Message-ID: <20250326133736.GF25239@noisy.programming.kicks-ass.net>
Date: Wed, 26 Mar 2025 14:37:36 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 20/21] futex: Implement FUTEX2_NUMA

On Wed, Mar 26, 2025 at 06:27:20PM +0530, Shrikanth Hegde wrote:
> 
> 
> On 3/26/25 04:26, Peter Zijlstra wrote:
> > On Wed, Mar 26, 2025 at 01:22:19AM +0530, Shrikanth Hegde wrote:
> > 
> > > > +	return &futex_queues[node][hash & futex_hashmask];
> > 
> >                              ^^^^^^^
> > 
> > > > +	hashsize = 256 * num_possible_cpus();
> > > > +	hashsize /= num_possible_nodes();
> > > 
> > > Wouldn't it be better to use num_online_nodes? each node may get a bigger
> > > hash bucket which means less collision no?
> > 
> > No. There are two problems with num_online_nodes, and both are evident
> > above.
> > 
> > Consider the case of a sparse set.
> 
> I am sorry, i didn't understand. Could you please explain?

I was confused; I should've just gone sleep :-)

The futex_queues[] array is sized MAX_NUMNODES, such that every possible
node_id has a spot. I thought we did dynamic sizing, but not so.

Anyway, using online here would lead to having to deal with hotplug,
which in turn either leads to more over-all hash buckets in the system,
or having to resize and rehash everything.

Neither are really attractive options.