Re: [PATCH v2] iio: trigger: Don't use RT priority

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 17 Sep 2020 14:03:33 +0200
Christian Eggers <ceggers@arri.de> wrote:

> Triggers may raise transactions on slow busses like I2C.  Using the
> original RT priority of a threaded IRQ may prevent other important IRQ
> handlers from being run.
> 
> Signed-off-by: Christian Eggers <ceggers@arri.de>
> Cc: stable@vger.kernel.org
> ---
> In my particular case (on a RT kernel), the RT priority of the sysfstrig
> threaded IRQ handler caused (temporarily) raising the prio of a user
> space process which was holding the I2C bus mutex.
> 
> Due to a bug in the i2c-imx driver, this process spent 500 ms in a busy-wait
> loop and prevented all threaded IRQ handlers from being run during this
> time.
I'm not sure I fully understand the impacts of this yet.

What is the impact on cases where we don't have any nasty side affects
due to users of the trigger?

I presume reducing the priority will cause some reduction in
performance?  If so is there any chance that would count as a regression?

Jonathan

> 
> v2:
> - Use sched_set_normal() instead of sched_setscheduler_nocheck()
> 
>  drivers/iio/industrialio-trigger.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/iio/industrialio-trigger.c b/drivers/iio/industrialio-trigger.c
> index 6f16357fd732..7ed00ad695c7 100644
> --- a/drivers/iio/industrialio-trigger.c
> +++ b/drivers/iio/industrialio-trigger.c
> @@ -9,7 +9,10 @@
>  #include <linux/err.h>
>  #include <linux/device.h>
>  #include <linux/interrupt.h>
> +#include <linux/irq.h>
> +#include <linux/irqdesc.h>
>  #include <linux/list.h>
> +#include <linux/sched.h>
>  #include <linux/slab.h>
>  
>  #include <linux/iio/iio.h>
> @@ -245,6 +248,7 @@ int iio_trigger_attach_poll_func(struct iio_trigger *trig,
>  	int ret = 0;
>  	bool notinuse
>  		= bitmap_empty(trig->pool, CONFIG_IIO_CONSUMERS_PER_TRIGGER);
> +	struct irq_desc *irq_desc;
>  
>  	/* Prevent the module from being removed whilst attached to a trigger */
>  	__module_get(pf->indio_dev->driver_module);
> @@ -264,6 +268,12 @@ int iio_trigger_attach_poll_func(struct iio_trigger *trig,
>  	if (ret < 0)
>  		goto out_put_irq;
>  
> +	/* Triggers may raise transactions on slow busses like I2C.  Using the original RT priority
> +	 * of a threaded IRQ may prevent other threaded IRQ handlers from being run.
> +	 */
> +	irq_desc = irq_to_desc(pf->irq);
> +	sched_set_normal(irq_desc->action->thread, 0);
> +
>  	/* Enable trigger in driver */
>  	if (trig->ops && trig->ops->set_trigger_state && notinuse) {
>  		ret = trig->ops->set_trigger_state(trig, true);