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

[Michal Hocko]

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