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Fri, 05 Sep 2025 02:09:05 -0700 (PDT) Received: from localhost.localdomain ([2a00:6d43:105:c401:e307:1a37:2e76:ce91]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45dda6da5casm12931995e9.7.2025.09.05.02.09.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 05 Sep 2025 02:09:05 -0700 (PDT) From: Marco Crivellari To: linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org Cc: Tejun Heo , Lai Jiangshan , Frederic Weisbecker , Sebastian Andrzej Siewior , Marco Crivellari , Michal Hocko , Alexander Gordeev , Vasily Gorbik , Heiko Carstens Subject: [PATCH 0/2] s390: replace wq users and add WQ_PERCPU to alloc_workqueue() users Date: Fri, 5 Sep 2025 11:08:55 +0200 Message-ID: <20250905090857.108240-1-marco.crivellari@suse.com> X-Mailer: git-send-email 2.51.0 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=UTF-8 Content-Transfer-Encoding: 8bit Below is a summary of a discussion about the Workqueue API and cpu isolation considerations. Details and more information are available here: "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND." https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/ === Current situation: problems === Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected. This leads to different scenarios if a work item is scheduled on an isolated CPU where "delay" value is 0 or greater then 0: schedule_delayed_work(, 0); This will be handled by __queue_work() that will queue the work item on the current local (isolated) CPU, while: schedule_delayed_work(, 1); Will move the timer on an housekeeping CPU, and schedule the work there. Currently if a user enqueue a work item using schedule_delayed_work() the used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to schedule_work() that is using system_wq and queue_work(), that makes use again of WORK_CPU_UNBOUND. This lack of consistentcy cannot be addressed without refactoring the API. === Plan and future plans === This patchset is the first stone on a refactoring needed in order to address the points aforementioned; it will have a positive impact also on the cpu isolation, in the long term, moving away percpu workqueue in favor to an unbound model. These are the main steps: 1) API refactoring (that this patch is introducing) - Make more clear and uniform the system wq names, both per-cpu and unbound. This to avoid any possible confusion on what should be used. - Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND, introduced in this patchset and used on all the callers that are not currently using WQ_UNBOUND. WQ_UNBOUND will be removed in a future release cycle. Most users don't need to be per-cpu, because they don't have locality requirements, because of that, a next future step will be make "unbound" the default behavior. 2) Check who really needs to be per-cpu - Remove the WQ_PERCPU flag when is not strictly required. 3) Add a new API (prefer local cpu) - There are users that don't require a local execution, like mentioned above; despite that, local execution yeld to performance gain. This new API will prefer the local execution, without requiring it. === Introduced Changes by this series === 1) [P 1] Replace use of system_wq system_wq is a per-CPU workqueue, but his name is not clear. Because of that, system_wq has been renamed in system_percpu_wq. 2) [P 2] add WQ_PERCPU to remaining alloc_workqueue() users Every alloc_workqueue() caller should use one among WQ_PERCPU or WQ_UNBOUND. This is actually enforced warning if both or none of them are present at the same time. WQ_UNBOUND will be removed in a next release cycle. === For Maintainers === There are prerequisites for this series, already merged in the master branch. The commits are: 128ea9f6ccfb6960293ae4212f4f97165e42222d ("workqueue: Add system_percpu_wq and system_dfl_wq") 930c2ea566aff59e962c50b2421d5fcc3b98b8be ("workqueue: Add new WQ_PERCPU flag") Thanks! Marco Crivellari (2): drivers/s390: WQ_PERCPU added to alloc_workqueue users s390: replace use of system_wq with system_percpu_wq arch/s390/kernel/diag/diag324.c | 4 ++-- arch/s390/kernel/hiperdispatch.c | 2 +- drivers/s390/char/tape_3590.c | 2 +- 3 files changed, 4 insertions(+), 4 deletions(-) -- 2.51.0