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

[Michal Hocko]

On Fri 20-02-26 18:58:14, Leonardo Bras wrote:
> On Mon, Feb 16, 2026 at 12:00:55PM +0100, Michal Hocko wrote:
> > 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. 
> 
> Awesome! Thanks for reviewing!
> 
> > 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.
> 
> That crossed my mind as well, and I went with the idea of changing locking 
> because I was working on workloads in which deferring work to a kernel 
> re-entry would cause deadline misses as well. Or more critically, the 
> drains could take forever, as some of those tasks would avoid returning to 
> kernel as much as possible. 

Could you be more specific please?
-- 
Michal Hocko
SUSE Labs