Re: ksoftirqd priority inversion wait_event_interruptible_timeout/schedule_timeout/del_timer_sync

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

On 2025-06-17 09:52:07 [+0200], Kegl Rohit wrote:
> Hello!
Hi,

> Using 5.10.184-rt90 on arm (nxp imx6q).
> 
> A proprietary driver starts a `kthread` with `SCHED_FIFO` -98 priority.
> This `kthread` waits for a completion using
> `wait_event_interruptible_timeout(comp, 1000)`
> and wakes up every second to perform its work. The completion itself
> only fires very infrequently.
> 
> Tracing the kernel showed that `ksoftirqd` was periodically boosted to
> the same `SCHED_FIFO` -98 priority.
> This is caused by the `schedule_timeout` handler within
> `wait_event_interruptible_timeout`.
> 
> del_timer_wait_running takes spin_lock_bh and causes the priority inversion.
> This is very bad because e.g. network softirq processing happens with
> very high priority for some time.
> 
> Was this issue addressed in a more recent kernel version?

The timer related bits moved to ktimersd thread (instead of ksoftirqd)
but the basic boosting problem remains.

There is work in progress to address this. The endgame is:
	https://lore.kernel.org/linux-rt-devel/20250613105653.1860729-1-bigeasy@linutronix.de/

Sebastian