linux-kernel - Re: [PATCH 0/4] Introduce QPW for per-cpu operations

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Message-ID: <40b42d83-3667-4489-818f-a01e1114d6b0@suse.cz>
Date: Wed, 11 Feb 2026 17:59:13 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Marcelo Tosatti <mtosatti@...hat.com>, Michal Hocko <mhocko@...e.com>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Shakeel Butt <shakeel.butt@...ux.dev>, Muchun Song <muchun.song@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>,
 Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>,
 Joonsoo Kim <iamjoonsoo.kim@....com>, Hyeonggon Yoo <42.hyeyoo@...il.com>,
 Leonardo Bras <leobras@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
 Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
 Frederic Weisbecker <fweisbecker@...e.de>
Subject: Re: [PATCH 0/4] Introduce QPW for per-cpu operations

On 2/11/26 17:50, Marcelo Tosatti wrote:
> On Wed, Feb 11, 2026 at 05:38:47PM +0100, Michal Hocko wrote:
>> On Wed 11-02-26 09:01:12, Marcelo Tosatti wrote:
>> > On Tue, Feb 10, 2026 at 03:01:10PM +0100, Michal Hocko wrote:
>> [...]
>> > > What about !PREEMPT_RT? We have people running isolated workloads and
>> > > these sorts of pcp disruptions are really unwelcome as well. They do not
>> > > have requirements as strong as RT workloads but the underlying
>> > > fundamental problem is the same. Frederic (now CCed) is working on
>> > > moving those pcp book keeping activities to be executed to the return to
>> > > the userspace which should be taking care of both RT and non-RT
>> > > configurations AFAICS.
>> > 
>> > Michal,
>> > 
>> > For !PREEMPT_RT, _if_ you select CONFIG_QPW=y, then there is a kernel
>> > boot option qpw=y/n, which controls whether the behaviour will be
>> > similar (the spinlock is taken on local_lock, similar to PREEMPT_RT).
>> 
>> My bad. I've misread the config space of this.
> 
> My bad, actually. Its only CONFIG_QPW on the current patchset.
> 
>> > If CONFIG_QPW=n, or kernel boot option qpw=n, then only local_lock 
>> > (and remote work via work_queue) is used.
>> > 
>> > What "pcp book keeping activities" you refer to ? I don't see how
>> > moving certain activities that happen under SLUB or LRU spinlocks
>> > to happen before return to userspace changes things related 
>> > to avoidance of CPU interruption ?
>> 
>> Essentially delayed operations like pcp state flushing happens on return
>> to the userspace on isolated CPUs. No locking changes are required as
>> the work is still per-cpu.
>> 
>> In other words the approach Frederic is working on is to not change the
>> locking of pcp delayed work but instead move that work into well defined
>> place - i.e. return to the userspace.
>> 
>> Btw. have you measure the impact of preempt_disbale -> spinlock on hot
>> paths like SLUB sheeves?
> 
> Nope, i have not. What is/are the standard benchmarks for SLUB/SLAB
> allocation ?

Those mentioned here, and I would say also netperf.
https://lore.kernel.org/all/20250913000935.1021068-1-sudarsanm@google.com/