Re: [PATCH net-next 3/3] net: WQ_PERCPU added to alloc_workqueue users

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Fri, Sep 05, 2025 at 11:05:05AM +0200, 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.
> 
> alloc_workqueue() treats all queues as per-CPU by default, while unbound
> workqueues must opt-in via WQ_UNBOUND.
> 
> This default is suboptimal: most workloads benefit from unbound queues,
> allowing the scheduler to place worker threads where they’re needed and
> reducing noise when CPUs are isolated.
> 
> This patch adds a new WQ_PERCPU flag at the network subsystem, to explicitly
> request the use of the per-CPU behavior. Both flags coexist for one release
> cycle to allow callers to transition their calls.
> 
> Once migration is complete, WQ_UNBOUND can be removed and unbound will
> become the implicit default.
> 
> With the introduction of the WQ_PERCPU flag (equivalent to !WQ_UNBOUND),
> any alloc_workqueue() caller that doesn’t explicitly specify WQ_UNBOUND
> must now use WQ_PERCPU.
> 
> All existing users have been updated accordingly.
> 
> Suggested-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>

[...]

> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index f0e48e6911fc..b3e960108e6b 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -916,7 +916,7 @@ static int __init virtio_vsock_init(void)
>  {
>  	int ret;
>  
> -	virtio_vsock_workqueue = alloc_workqueue("virtio_vsock", 0, 0);
> +	virtio_vsock_workqueue = alloc_workqueue("virtio_vsock", WQ_PERCPU, 0);
>  	if (!virtio_vsock_workqueue)
>  		return -ENOMEM;
>  
> diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
> index 6e78927a598e..bc2ff918b315 100644
> --- a/net/vmw_vsock/vsock_loopback.c
> +++ b/net/vmw_vsock/vsock_loopback.c
> @@ -139,7 +139,7 @@ static int __init vsock_loopback_init(void)
>  	struct vsock_loopback *vsock = &the_vsock_loopback;
>  	int ret;
>  
> -	vsock->workqueue = alloc_workqueue("vsock-loopback", 0, 0);
> +	vsock->workqueue = alloc_workqueue("vsock-loopback", WQ_PERCPU, 0);
>  	if (!vsock->workqueue)
>  		return -ENOMEM;
>  
 
LGTM for the vmw_vsock bits. Regarding step 2 "Check who really needs to
be per-cpu", IIRC a few years ago I did some playing around with per-cpu
wq for vsock and I don't think I saw a huge difference in performance,
so I'd expect it to be in the "not really needs per-cpu" camp... I might
be able to help re-evaluate that when the time comes.

Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>