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Fri, 05 Sep 2025 02:03:32 -0700 (PDT) Received: from localhost.localdomain ([2a00:6d43:105:c401:e307:1a37:2e76:ce91]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3cf3458a67fsm3614543f8f.62.2025.09.05.02.03.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 05 Sep 2025 02:03:31 -0700 (PDT) From: Marco Crivellari To: linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: Tejun Heo , Lai Jiangshan , Frederic Weisbecker , Sebastian Andrzej Siewior , Marco Crivellari , Michal Hocko , Andrew Morton Subject: [PATCH 0/3] mm: replace wq users and add WQ_PERCPU to alloc_workqueue() users Date: Fri, 5 Sep 2025 11:03:20 +0200 Message-ID: <20250905090323.103401-1-marco.crivellari@suse.com> X-Mailer: git-send-email 2.51.0 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: DC90220005 X-Stat-Signature: 6pqq4q5qgqw417b7ikco3kmacssf5ed9 X-Rspam-User: X-HE-Tag: 1757063013-327182 X-HE-Meta: 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 E0UmjVOR rU0zKlV4sO8WyC1g3+ZmmwKJyCvO1M1x20DzMyXnKeLH4arDi0/lJxjzWbv9jQ8+ZzK47YAa8DKkVjRKsu1GD0MWvrlkrhQ+Tl7sLlElj1XLvlLMmcgdz8/aQa8fqrzYO7lyMCn/u2BNbGBCphtJCc2l6DSaOhX5mUILdm5nI0hDGWVzGd1Y1/TPQ+nBdRAaoA0+qh13ohFivEYjaGgQmrSb0SJRfkFY4itAIVo2NydFOJ4Q6fSCPXf8iGP7+yyiHsb9BhUH6q/crDyH+yveR/INXq78RWdiGKh1/7hlaYcwcZdUX3TMrS3GkL7f6vf5mplxYxQDUKyLVFGS4HZAur+uynJ/zKItxytHDf7vGfo5CoxGLjyxrf/SdS8o8T6A3K6Th 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: Hi! 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 1-2] Replace use of system_wq and system_unbound_wq system_wq is a per-CPU workqueue, but his name is not clear. system_unbound_wq is to be used when locality is not required. Because of that, system_wq has been renamed in system_percpu_wq, and system_unbound_wq has been renamed in system_dfl_wq. 2) [P 3] 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 (3): mm: replace use of system_unbound_wq with system_dfl_wq mm: replace use of system_wq with system_percpu_wq mm: WQ_PERCPU added to alloc_workqueue users mm/backing-dev.c | 6 +++--- mm/kfence/core.c | 6 +++--- mm/memcontrol.c | 4 ++-- mm/slub.c | 3 ++- mm/vmstat.c | 3 ++- 5 files changed, 12 insertions(+), 10 deletions(-) -- 2.51.0