linux-kernel - Re: [RFC PATCH 0/4] Optimize rss_stat initialization/teardown for single-threaded tasks

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Message-ID: <bcc7140f-5efb-481c-b1e9-564a2d88ed81@efficios.com>
Date: Fri, 28 Nov 2025 15:12:25 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Jan Kara <jack@...e.cz>
Cc: Gabriel Krisman Bertazi <krisman@...e.de>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Mateusz Guzik <mjguzik@...il.com>,
 Shakeel Butt <shakeel.butt@...ux.dev>, Michal Hocko <mhocko@...nel.org>,
 Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
 Christoph Lameter <cl@...two.org>, Andrew Morton
 <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 0/4] Optimize rss_stat initialization/teardown for
 single-threaded tasks

On 2025-11-28 15:10, Jan Kara wrote:
> On Fri 28-11-25 08:30:08, Mathieu Desnoyers wrote:
[...]
>> I suspect that by doing just that we'd get most of the
>> performance benefits provided by the single-threaded special-case
>> proposed here.
> 
> I don't think so. Because in the profiles I have been doing for these
> loads the biggest cost wasn't actually the per-cpu allocation itself but
> the cost of zeroing the allocated counter for many CPUs (and then the
> counter summarization on exit) and you're not going to get rid of that with
> just reshuffling per-cpu fields and adding slab allocator in front.

That's a good point ! So skipping the zeroing of per-cpu fields would
indeed justify special-casing the single-threaded case.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com