Marco Crivellari <marco.crivellari@xxxxxxxx> wrote: > 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@xxxxxxxxxxxxx/ > > === 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_unbound_wq > > system_unbound_wq is to be used when locality is not required. > > Because of that, system_unbound_wq has been renamed in system_dfl_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): > padata: replace use of system_unbound_wq with system_dfl_wq > padata: WQ_PERCPU added to alloc_workqueue users > > kernel/padata.c | 9 +++++---- > 1 file changed, 5 insertions(+), 4 deletions(-) All applied. Thanks. -- Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
