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

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Message-ID: <20251229164059.GA65997@bhelgaas>
Date: Mon, 29 Dec 2025 10:40:59 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-pci@...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>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Ilpo Jarvinen <ilpo.jarvinen@...ux.intel.com>,
	Manivannan Sadhasivam <mani@...nel.org>
Subject: Re: [PATCH] PCI: shpchp: add WQ_PERCPU to alloc_workqueue users

[+cc Mani]

On Fri, Nov 07, 2025 at 03:36:24PM +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>

Squashed with similar patches [1] and applied on pci/workqueue for
v6.20, thanks!

See https://lore.kernel.org/r/20251229163858.GA63361@bhelgaas

> ---
>  drivers/pci/hotplug/shpchp_core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/hotplug/shpchp_core.c b/drivers/pci/hotplug/shpchp_core.c
> index 0c341453afc6..56308515ecba 100644
> --- a/drivers/pci/hotplug/shpchp_core.c
> +++ b/drivers/pci/hotplug/shpchp_core.c
> @@ -80,7 +80,8 @@ static int init_slots(struct controller *ctrl)
>  		slot->device = ctrl->slot_device_offset + i;
>  		slot->number = ctrl->first_slot + (ctrl->slot_num_inc * i);
>  
> -		slot->wq = alloc_workqueue("shpchp-%d", 0, 0, slot->number);
> +		slot->wq = alloc_workqueue("shpchp-%d", WQ_PERCPU, 0,
> +					   slot->number);
>  		if (!slot->wq) {
>  			retval = -ENOMEM;
>  			goto error_slot;
> -- 
> 2.51.1
>