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

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Message-ID: <aYywl1hdBQP2_slo@tiehlicka>
Date: Wed, 11 Feb 2026 17:38:47 +0100
From: Michal Hocko <mhocko@...e.com>
To: Marcelo Tosatti <mtosatti@...hat.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>,
	Vlastimil Babka <vbabka@...e.cz>,
	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 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.

> 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?
-- 
Michal Hocko
SUSE Labs