Seeking input on beginner-level bugs related to preempt_RT

Seeking input on beginner-level bugs related to preempt_RT

[Miguel Miranda]

Hello everyone,

I am currently looking for beginner-level bugs related to preempt_RT for 
the Linux kernel Bug Fixing Spring Unpaid 2024. Although I am not an 
expert in RT, I have been trying to identify some bugs and our mentors 
have suggested using ring testers such as syzkaller and test frameworks 
like kernelselftests to find and reproduce these bugs on our own.

I have a few questions and would appreciate any input you may have. 
Firstly, is there a specific CI ring or channel that lists KTODOs, or a 
preferred platform where bugs related to PREEMPT_RT are reported? 
Secondly, I have been following the LKML and have seen some 
console_blast.sh results, but I am not sure where to find this 
console_blast. Lastly, I have come across some recent discussions about 
printk() related work, but I'm unsure if there is a list to avoid rework.

Thank you for your time.

Re: Seeking input on beginner-level bugs related to preempt_RT

[John B. Wyatt IV]

On Fri, Apr 12, 2024 at 05:00:39PM -0600, Miguel Miranda wrote:
> Hello everyone,
> 

Welcome.

> I am currently looking for beginner-level bugs related to preempt_RT for the
> Linux kernel Bug Fixing Spring Unpaid 2024.

What is your experience level? Have you built and run an rt kernel
before? (Selected Fully Preemptive in kconfig?)

> Although I am not an expert in
> RT, I have been trying to identify some bugs and our mentors have suggested
> using ring testers such as syzkaller and test frameworks like
> kernelselftests to find and reproduce these bugs on our own.

A lot of the bugs we encounter are around locking. RT changes
the way locks are handled in the kernel. That is one avenue I would
encourage you to study.

Not sure if anyone has actually used syzkaller on the preempt-rt
patchset. Never used it before, but I would be interested in it's
results.

You can read about kselftests here:

https://elixir.bootlin.com/linux/latest/source/Documentation/dev-tools/kselftest.rst

> 
> I have a few questions and would appreciate any input you may have. Firstly,
> is there a specific CI ring or channel that lists KTODOs, or a preferred
> platform where bugs related to PREEMPT_RT are reported? Secondly, I have
> been following the LKML and have seen some console_blast.sh results, but I
> am not sure where to find this console_blast. Lastly, I have come across
> some recent discussions about printk() related work, but I'm unsure if there
> is a list to avoid rework.

John Ogness sent me that script and I posted in another email on this
list. I forget which one.

John specifically asked for a very large core machine to do the testing
on with a uart 8250 interface. That might be difficult for you to
procure but I have included it below. Please make sure to use the
correct line endings or Bash will not like it. :)

#!/bin/sh
set -e

# This script is intended to be started with no arguments.
#
# It then calls itself to create a pinned process for each CPU. Those
# child processes will run in infinite loops of show-task-states via
# /proc/sysrq-trigger. This generates lots of contention on the console.
#
# Finally, it waits 10 seconds before crashing the system.

if [ $# -eq 0 ]; then
	cpus=$(($(nproc) - 1))
	for i in $(seq 0 $cpus); do
		$0 $i &
	done

	sleep 15
	echo c > /proc/sysrq-trigger
	exit 0
fi

# Setup the CPU mask for the provided CPU number. The real taskset(1)
# could simply use -c, but the busybox(1) variant does not support this.
mask=$(printf "0x%x\n" $((1 << $1)))
taskset -p $mask $$

# Wait a moment before beginning blast.
sleep 2

while true; do
	echo t > /proc/sysrq-trigger
done


-- 
Sincerely,
John Wyatt
Software Engineer, Core Kernel
Red Hat

Re: Seeking input on beginner-level bugs related to preempt_RT

[Sebastian Andrzej Siewior]

On 2024-04-17 13:42:03 [-0400], John B. Wyatt IV wrote:
> On Fri, Apr 12, 2024 at 05:00:39PM -0600, Miguel Miranda wrote:
> > Hello everyone,
> > 
> 
> Welcome.
Hi,

> > I am currently looking for beginner-level bugs related to preempt_RT for the
> > Linux kernel Bug Fixing Spring Unpaid 2024.
> 
> What is your experience level? Have you built and run an rt kernel
> before? (Selected Fully Preemptive in kconfig?)
> 
> > Although I am not an expert in
> > RT, I have been trying to identify some bugs and our mentors have suggested
> > using ring testers such as syzkaller and test frameworks like
> > kernelselftests to find and reproduce these bugs on our own.
> 
> A lot of the bugs we encounter are around locking. RT changes
> the way locks are handled in the kernel. That is one avenue I would
> encourage you to study.

I would suggest to build the PREEMPT_RT kernel and starting using on a
wide range of hardware and seeing how it behaves.

Sebastian