Re: [PATCH] uio: eliminate extraneous irq thread under CONFIG_PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

Chris, what was the use case to split this?

On 2026-08-19 15:01:00 [+0000], Mike Schanne wrote:
> Commit f8a27dfa4b82d442af1c0645a5acc70cc97c67f6 ("uio: use threaded
> interrupts") makes all uio interrupts threaded by default.  However
> under CONFIG_PREEMPT_RT, all interrupts are already threaded.  This
> results in 2 irq threads per uio instance, adding an unnecessary
> extra context switch per interrupt.
> 
> Do not request a threaded interrupt for uio under CONFIG_PREEMPT_RT
> 
> Signed-off-by: Michael Schanne mschanne@kns.com<mailto:mschanne@kns.com>
> ---
> drivers/uio/uio.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
> index 004a549c6..4e28777ac 100644
> --- a/drivers/uio/uio.c
> +++ b/drivers/uio/uio.c
> @@ -448,8 +448,12 @@ static irqreturn_t uio_interrupt_handler(int irq, void *dev_id)
>                irqreturn_t ret;
>                 ret = idev->info->handler(irq, idev->info);
> -              if (ret == IRQ_HANDLED)
> -                              ret = IRQ_WAKE_THREAD;
> +             if (ret == IRQ_HANDLED) {
> +                             if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +                                             uio_event_notify(idev->info);
> +                             else
> +                                             ret = IRQ_WAKE_THREAD;
> +             }
>                 return ret;
> }
> @@ -1038,8 +1042,13 @@ int __uio_register_device(struct module *owner,
>                                 * FDs at the time of unregister and therefore may not be
>                                 * freed until they are released.
>                                 */
> -                              ret = request_threaded_irq(info->irq, uio_interrupt_handler, uio_interrupt_thread,
> -                                                                                 info->irq_flags, info->name, idev);
> +                             if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +                                             ret = request_irq(info->irq, uio_interrupt_handler,
> +                                                             info->irq_flags, info->name, idev);
> +                             } else {
> +                                             ret = request_threaded_irq(info->irq, uio_interrupt_handler, uio_interrupt_thread,
> +                                                             info->irq_flags, info->name, idev);
> +                             }
>                                if (ret) {
>                                                info->uio_dev = NULL;
>                                                goto err_request_irq;

Sebastian