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

[Chris Packham]

Hi Sebastian,

On 20/08/2026 03:06, Sebastian Andrzej Siewior wrote:
> Chris, what was the use case to split this?

I was dealing with having a UIO interrupt for an input on a PCA9539. 
There's some more info on the original email thread[1] but basically 
because the PCA9539 inputs are nested __setup_irq() enforces that anyone 
requesting them does so with a threaded interrupt.

https://lore.kernel.org/all/20240408234050.2056374-3-chris.packham@xxxxxxxxxxxxxxxxxxx/

>
> 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@xxxxxxx<mailto:mschanne@xxxxxxx>
>> ---
>> 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