Re: [RFC PATCH] sched: add CONFIG_SCHED_EXT_RT_EXPERIMENTAL to move SCHED_EXT above SCHED_RR/SCHED_FIFO

[Tejun Heo <tj@kernel.org>]

Hello,

On Tue, Apr 14, 2026 at 02:59:33PM +0200, Fredrik Boettger Bratberg wrote:
> A recent paper published in the 2025 Real-Time Systems Symposium (RTSS)
> "Enabling Flexible Scheduling in ROS 2", would presumably also benefint
> from this feature. The paper aims to address and improve the real-time
> capabilities of Robot Operating System 2 (ROS2). As part of their paper
> they implement an EDF using sched_ext. They do not explicitly address the
> issue of their EDF SCHED_EXT tasks effectively having a lower priority than
> SCHED_NORMAL, but it is a known limitation of the current sched_ext class
> placement

Isn't that mostly solvable by just not putting tasks in DL/RT and enabling
switching_all?

> This change conditionally reorders sched_class selection such that, when
> enabled:
> 
>   STOP > DL > EXT > RT > FAIR > IDLE

The problem is that RT being above EXT is required for safety as EXT depends
on RT superceding it for enable/disable path forward progress guarantee. If
you put EXT on top of RT, it's relatively easy to stall out the whole system
without any way to recover it.

Thanks.

-- 
tejun