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

[Marcelo Tosatti]

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.
> 
> > 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.

Michal,

I can't find such work from Frederic. Do you mean:

Subject: Re: [PATCH v6 00/29] context_tracking,x86: Defer some IPIs until a user->kernel transition

>From Valentin ?

Or if you have a pointer to Frederic's work.

> Btw. have you measure the impact of preempt_disbale -> spinlock on hot
> paths like SLUB sheeves?

Doing that, will post results as soon as possible.