linux-xilinx-zynqmp/focal: main(): sched_setscheduler(): Operation not permitted

[Portia Stephens]

[SRU Justification]

[Impact]
The focal kernel is failing the ubuntu_ltp:sched:time-schedule01, ubuntu_ltp:sched:trace_sched01 and ubuntu_ltp:irq:irqbalance0 regression test due to sched_setscheduler being unable to a task class to SCHED_FIFO. This is due to have having CONFIG_RT_GROUP_SCHED enabled. Once RT_GROUP_SCHED is enabled it is impossible to schedule realtime tasks for non-root users until you allocate realtime bandwidth for them.

There are multiple known issues with CONFIG_RT_GROUP_SCHED that are reported in the debian.master/config/annotations file:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1284731
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1873315
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1875665

[Fix]

CONFIG_RT_GROUP_SCHED should be disabled. The config was enabled when we copied the Xilinx default config settings and there is not a justification for this exact config. There may even be a negative impact, from the kernel.org documentation:

“The default values for sched_rt_period_us (1000000 or 1s) and
sched_rt_runtime_us (950000 or 0.95s). This gives 0.05s to be used by
SCHED_OTHER (non-RT tasks).”

This suggests this option will benefit RT systems, but be detrimental to non-RT systems,

[Testcase]

The annotations file will enforce that CONFIG_RT_GROUP_SCHED is disabled.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-xilinx-zynqmp/5.4.0-1020.24 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-focal' to 'verification-done-focal'. If the problem still exists, change the tag 'verification-needed-focal' to 'verification-failed-focal'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!