Re: [PATCH 1/1] module: replace use of system_wq with system_percpu_wq

[Petr Pavlu <petr.pavlu@suse.com>]

On 9/5/25 11:01 AM, Marco Crivellari wrote:
> 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.
> 
> system_wq is a per-CPU worqueue, yet nothing in its name tells about that
> CPU affinity constraint, which is very often not required by users. Make
> it clear by adding a system_percpu_wq.
> 
> queue_work() / queue_delayed_work() mod_delayed_work() will now use the
> new per-cpu wq: whether the user still stick on the old name a warn will
> be printed along a wq redirect to the new one.
> 
> This patch add the new system_percpu_wq except for mm, fs and net
> subsystem, whom are handled in separated patches.
> 
> The old wq will be kept for a few release cylces.
> 
> Suggested-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
> ---
>  kernel/module/dups.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/module/dups.c b/kernel/module/dups.c
> index bd2149fbe117..e72fa393a2ec 100644
> --- a/kernel/module/dups.c
> +++ b/kernel/module/dups.c
> @@ -113,7 +113,7 @@ static void kmod_dup_request_complete(struct work_struct *work)
>  	 * let this linger forever as this is just a boot optimization for
>  	 * possible abuses of vmalloc() incurred by finit_module() thrashing.
>  	 */
> -	queue_delayed_work(system_wq, &kmod_req->delete_work, 60 * HZ);
> +	queue_delayed_work(system_percpu_wq, &kmod_req->delete_work, 60 * HZ);
>  }
>  
>  bool kmod_dup_request_exists_wait(char *module_name, bool wait, int *dup_ret)
> @@ -240,7 +240,7 @@ void kmod_dup_request_announce(char *module_name, int ret)
>  	 * There is no rush. But we also don't want to hold the
>  	 * caller up forever or introduce any boot delays.
>  	 */
> -	queue_work(system_wq, &kmod_req->complete_work);
> +	queue_work(system_percpu_wq, &kmod_req->complete_work);
>  
>  out:
>  	mutex_unlock(&kmod_dup_mutex);

The two work items queued by the dups.c code can run anywhere. I don't
see a reason why they need to be bound to a specific CPU.

If I understand the cover letter and its linked discussion correctly,
the aim is to eventually move users to unbound workqueues unless they
really need to use per-CPU workqueues.

If it helps, I believe you can already update this code to use the new
system_dfl_wq.

-- 
Thanks,
Petr