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

[Michal Hocko <mhocko@suse.com>]

On Sat 14-02-26 19:02:19, Leonardo Bras 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.
> > 
> > > 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?
> 
> Hi Michal,
> 
> I have done some study on this (which I presented on Plumbers 2023):
> https://lpc.events/event/17/contributions/1484/ 
> 
> Since they are per-cpu spinlocks, and the remote operations are not that 
> frequent, as per design of the current approach, we are not supposed to see 
> contention (I was not able to detect contention even after stress testing 
> for weeks), nor relevant cacheline bouncing.
> 
> That being said, for RT local_locks already get per-cpu spinlocks, so there 
> is only difference for !RT, which as you mention, does preemtp_disable():
> 
> The performance impact noticed was mostly about jumping around in 
> executable code, as inlining spinlocks (test #2 on presentation) took care 
> of most of the added extra cycles, adding about 4-14 extra cycles per 
> lock/unlock cycle. (tested on memcg with kmalloc test)
> 
> Yeah, as expected there is some extra cycles, as we are doing extra atomic 
> operations (even if in a local cacheline) in !RT case, but this could be 
> enabled only if the user thinks this is an ok cost for reducing 
> interruptions.
> 
> What do you think?

The fact that the behavior is opt-in for !RT is certainly a plus. I also
do not expect the overhead to be really be really big. To me, a much
more important question is which of the two approaches is easier to
maintain long term. The pcp work needs to be done one way or the other.
Whether we want to tweak locking or do it at a very well defined time is
the bigger question.
-- 
Michal Hocko
SUSE Labs