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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: 6m51a7qkkz14y1qpg8w9mhu1kyxsuxuy X-Rspamd-Queue-Id: F051640004 X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1770827930-23006 X-HE-Meta: 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 nbnN40Tt 2E2XmpPmOO50jKgYz4rZ53Z3on2F9iX61O3mucbKuIbwC7aZu2dW6qgUc37fXTSgRC/XGfnd1X60TqT8RiL61c/T1JCaYsPa+mqtYkhtn4F54ZLTGIX7f9gTE9Wl2+jL3r0K31Y/4FiNJvsAk5VzTLCXNYKP2bhwpnLDEMPNTnS9Hiqlp6WDJ7D+f2B7Cyc1Zzqoig3WH+0pDYSzdoUr4hPeYWGAgIdxu57KwqvBj/u+x98IpZGkGNetg9eEuGxlJCxALh22TQ8u6jNY8NZ1no5anMddAwnWgGVIBjQbi0NRPdChnpyz7RNwP8u8AiHPvqY6k7qRULjsCxrE5DB3DE2TxPDdz4Unw80LZwlGk6X2/WtFo0X0OqRrnz40XEttR6YBMy8rIiqalTJlBDFxHsScfLoVrCp2zl5wahLWqwVTAdUk= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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