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

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Message-ID: <4d0059f1-d834-44d5-bc57-7425980f5a97@linux.ibm.com>
Date: Wed, 26 Mar 2025 18:27:20 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: 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 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?