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

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Message-ID: <20250325225600.GP36322@noisy.programming.kicks-ass.net>
Date: Tue, 25 Mar 2025 23:56:00 +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 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.