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

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Message-ID: <20251229163858.GA63361@bhelgaas>
Date: Mon, 29 Dec 2025 10:38:58 -0600
From: Bjorn Helgaas <healgaas@...nel.org>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.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>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH] PCI: pnv_php: add WQ_PERCPU to alloc_workqueue users

On Fri, Nov 07, 2025 at 03:33:35PM +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.

Thanks for these patches.  We have:

  [PATCH] PCI: endpoint: add WQ_PERCPU to alloc_workqueue users
  [PATCH] PCI: endpoint: epf-mhi: add WQ_PERCPU to alloc_workqueue users
  [PATCH] PCI: endpoint: pci-epf-test: add WQ_PERCPU to alloc_workqueue users
  [PATCH] PCI: pnv_php: add WQ_PERCPU to alloc_workqueue users
  [PATCH] PCI: shpchp: add WQ_PERCPU to alloc_workqueue users
  [PATCH] PCI: endpoint: replace use of system_wq with system_percpu_wq

IIUC these are all part of the same effort to refactor the workqueue
API and don't really have anything to do with the endpoint or hotplug
drivers themselves.

So I put these all on pci/workqueue and squashed the WQ_PERCPU patches
together because they do the same thing, they have the same commit
log, and there's not really any point in reviewing them separately.

> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
> ---
>  drivers/pci/hotplug/pnv_php.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/hotplug/pnv_php.c b/drivers/pci/hotplug/pnv_php.c
> index c5345bff9a55..35f1758126c6 100644
> --- a/drivers/pci/hotplug/pnv_php.c
> +++ b/drivers/pci/hotplug/pnv_php.c
> @@ -802,7 +802,7 @@ static struct pnv_php_slot *pnv_php_alloc_slot(struct device_node *dn)
>  	}
>  
>  	/* Allocate workqueue for this slot's interrupt handling */
> -	php_slot->wq = alloc_workqueue("pciehp-%s", 0, 0, php_slot->name);
> +	php_slot->wq = alloc_workqueue("pciehp-%s", WQ_PERCPU, 0, php_slot->name);
>  	if (!php_slot->wq) {
>  		SLOT_WARN(php_slot, "Cannot alloc workqueue\n");
>  		kfree(php_slot->name);
> -- 
> 2.51.1
>