Re: [PATCH] media: platform: mtk-mdp3: add WQ_PERCPU to alloc_workqueue users

[Nicolas Dufresne <nicolas@ndufresne.ca>]

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Hi,

Le vendredi 07 novembre 2025 à 15:13 +0100, Marco Crivellari a écrit :
> 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.

I have to admit, there is likely no review here due to the lack of knowledge, so
in order to help educate myself (hopefully its not just me), can you explain why
the new default of WQ_UNBOUND would not be a fit for this driver ? After all,
the author didn't care and didn't make a choice, so I feel like its worth
asking.

cheers,
Nicolas

> 
> Suggested-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
> ---
>  drivers/media/platform/mediatek/mdp3/mtk-mdp3-core.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/platform/mediatek/mdp3/mtk-mdp3-core.c
> b/drivers/media/platform/mediatek/mdp3/mtk-mdp3-core.c
> index 6559d72d5d42..9083367ae2e4 100644
> --- a/drivers/media/platform/mediatek/mdp3/mtk-mdp3-core.c
> +++ b/drivers/media/platform/mediatek/mdp3/mtk-mdp3-core.c
> @@ -255,14 +255,16 @@ static int mdp_probe(struct platform_device *pdev)
>  		goto err_free_mutex;
>  	}
>  
> -	mdp->job_wq = alloc_workqueue(MDP_MODULE_NAME, WQ_FREEZABLE, 0);
> +	mdp->job_wq = alloc_workqueue(MDP_MODULE_NAME,
> +				      WQ_FREEZABLE | WQ_PERCPU, 0);
>  	if (!mdp->job_wq) {
>  		dev_err(dev, "Unable to create job workqueue\n");
>  		ret = -ENOMEM;
>  		goto err_deinit_comp;
>  	}
>  
> -	mdp->clock_wq = alloc_workqueue(MDP_MODULE_NAME "-clock",
> WQ_FREEZABLE,
> +	mdp->clock_wq = alloc_workqueue(MDP_MODULE_NAME "-clock",
> +					WQ_FREEZABLE | WQ_PERCPU,
>  					0);
>  	if (!mdp->clock_wq) {
>  		dev_err(dev, "Unable to create clock workqueue\n");

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