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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <89bb1d0e-4835-43d4-9e63-0b1717a9921e@linux.ibm.com>
Date: Wed, 26 Mar 2025 20:36:08 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
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 3/26/25 19:07, Peter Zijlstra wrote:
> 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.
> 

Ok. got it. Thanks.
Keeping with possible nodes seems simpler.