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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Subject: [PATCH 0/2] Workqueue: net: replace use of system_wq and add WQ_PERCPU to alloc_workqueue users Date: Fri, 15 Aug 2025 11:48:06 +0200 Message-ID: <20250815094808.54888-1-marco.crivellari@suse.com> X-Mailer: git-send-email 2.50.1 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 Hello! 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 net) 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 net subsystm (details in the next section). 2) [P 2] add WQ_PERCPU to alloc_workqueue() users (under net) 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 net subsystem (details in the next section). WQ_UNBOUND will be removed in a next release cycle. === For net 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: net: replace use of system_wq with system_percpu_wq Workqueue: net: WQ_PERCPU added to alloc_workqueue users net/bridge/br_cfm.c | 6 +++--- net/bridge/br_mrp.c | 8 ++++---- net/ceph/messenger.c | 3 ++- net/ceph/mon_client.c | 2 +- net/core/skmsg.c | 2 +- net/core/sock_diag.c | 2 +- net/devlink/core.c | 2 +- net/ipv4/inet_fragment.c | 2 +- net/netfilter/nf_conntrack_ecache.c | 2 +- net/openvswitch/dp_notify.c | 2 +- net/rds/ib_rdma.c | 3 ++- net/rfkill/input.c | 2 +- net/rxrpc/rxperf.c | 2 +- net/smc/af_smc.c | 6 +++--- net/smc/smc_core.c | 4 ++-- net/tls/tls_device.c | 2 +- net/vmw_vsock/af_vsock.c | 2 +- net/vmw_vsock/virtio_transport.c | 2 +- net/vmw_vsock/vsock_loopback.c | 2 +- 19 files changed, 29 insertions(+), 27 deletions(-) -- 2.50.1