Re: [PATCH] kernel:irq:manage: request threaded irq with a specified priority

[Thomas Gleixner]

On Tue, Apr 13 2021 at 14:19, Song Chen wrote:
> In general, irq handler thread will be assigned a default priority which
> is MAX_RT_PRIO/2, as a result, no one can preempt others.
>
> Here is the case I found in a real project, an interrupt int_a is
> coming, wakes up its handler handler_a and handler_a wakes up a
> userspace RT process task_a.
>
> However, if another irq handler handler_b which has nothing to do
> with any RT tasks is running when int_a is coming, handler_a can't
> preempt handler_b, as a result, task_a can't be waken up immediately
> as expected until handler_b gives up cpu voluntarily. In this case,
> determinism breaks.

It breaks because the system designer failed to assign proper priorities
to the irq threads int_a, int_b and to the user space process task_a.

That's not solvable at the kernel level.

Thanks,

        tglx