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[109.81.83.241]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4835a5bf1efsm47887645e9.0.2026.02.11.08.38.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Feb 2026 08:38:48 -0800 (PST) Date: Wed, 11 Feb 2026 17:38:47 +0100 From: Michal Hocko To: Marcelo Tosatti Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Leonardo Bras , Thomas Gleixner , Waiman Long , Boqun Feng , Frederic Weisbecker Subject: Re: [PATCH 0/4] Introduce QPW for per-cpu operations Message-ID: References: <20260206143430.021026873@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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