linux-kernel - Re: [PATCH] mshv_eventfd: add WQ_PERCPU to alloc

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Message-ID: <20251107181156.GA4041739@liuwe-devbox-debian-v2.local>
Date: Fri, 7 Nov 2025 18:11:56 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Michal Hocko <mhocko@...e.com>,
	"K . Y . Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>
Subject: Re: [PATCH] mshv_eventfd: add WQ_PERCPU to alloc_workqueue users

On Fri, Nov 07, 2025 at 02:27:12PM +0100, Marco Crivellari wrote:
> Currently if a user enqueues 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 consistency 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 continues the effort to refactor workqueue APIs, which began with
> the introduction of new workqueues and a new alloc_workqueue flag in:
> 
> commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
> commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")
> 
> This change adds a new WQ_PERCPU flag to explicitly request
> alloc_workqueue() to be per-cpu when WQ_UNBOUND has not been specified.
> 
> 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.
> 
> Once migration is complete, WQ_UNBOUND can be removed and unbound will
> become the implicit default.
> 
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>

Applied to hyperv-next. Thanks.

I modified the subject prefix to "mshv" and added a new line in the
commit message body to separate the first two paragraphs while at it.

Wei

> ---
>  drivers/hv/mshv_eventfd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/mshv_eventfd.c b/drivers/hv/mshv_eventfd.c
> index 806674722868..2a80af1d610a 100644
> --- a/drivers/hv/mshv_eventfd.c
> +++ b/drivers/hv/mshv_eventfd.c
> @@ -592,7 +592,7 @@ static void mshv_irqfd_release(struct mshv_partition *pt)
>  
>  int mshv_irqfd_wq_init(void)
>  {
> -	irqfd_cleanup_wq = alloc_workqueue("mshv-irqfd-cleanup", 0, 0);
> +	irqfd_cleanup_wq = alloc_workqueue("mshv-irqfd-cleanup", WQ_PERCPU, 0);
>  	if (!irqfd_cleanup_wq)
>  		return -ENOMEM;
>  
> -- 
> 2.51.1
>