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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 patchset === 1) [P 1] replace use of system_wq with system_percpu_wq (under mm) 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 in the mm subsystm (details in the next section). 2) [P 2] add WQ_PERCPU to alloc_workqueue() users (under mm) 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. These patches introduce WQ_PERCPU in the mm subsystem (details in the next section). WQ_UNBOUND will be removed in a next release cycle. === For mm Maintainers === If you agree with these changes, one option is pull the preparation changes from Tejun's wq branch [1]. As an alternative, the patches can be routed through a wq branch. The preparation changes are described in the present cover letter, under the "main steps" section. The changes done in summary are: - add system_percpu_wq and system_dfl_wq, for now without replace the older wq(s) (system_unbound_wq and system_wq). - add WQ_PERCPU flag, currently without removing WQ_UNBOUND; it will be removed in a future release cycle. You can find the aforementioned changes reading: ("Workqueue: add WQ_PERCPU, system_dfl_wq and system_percpu_wq") https://lore.kernel.org/all/20250614133531.76742-1-marco.crivellari@suse.com/ - [1] git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git WQ_PERCPU Thanks! Marco Crivellari (2): Workqueue: mm: replace use of system_wq with system_percpu_wq Workqueue: mm: WQ_PERCPU added to alloc_workqueue users mm/backing-dev.c | 4 ++-- mm/slub.c | 3 ++- mm/vmstat.c | 3 ++- 3 files changed, 6 insertions(+), 4 deletions(-) -- 2.50.1