linux-kernel - Re: [RFC} Commit 8a99b6833c88 Moves Important Real-time Settings To DebugFS

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Date:   Fri, 19 Aug 2022 10:28:44 -0400
From:   Joseph Salisbury <joseph.salisbury@...onical.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-rt-users@...r.kernel.org, williams@...hat.com,
        bigeasy@...utronix.de, valentin.schneider@....com,
        linux-kernel@...r.kernel.org,
        Marcelo Cerri <marcelo.cerri@...onical.com>
Subject: Re: [RFC} Commit 8a99b6833c88 Moves Important Real-time Settings To
 DebugFS



On 8/18/22 21:38, Joseph Salisbury wrote:
>
>
> On 8/18/22 16:31, Peter Zijlstra wrote:
>> On Thu, Aug 18, 2022 at 03:53:28PM -0400, Joseph Salisbury wrote:
>>> Hello,
>>>
>>> Some Ubuntu users are using the tuned package with a 5.15.x based 
>>> real-time
>>> kernel.  Tuned adjusts various sysctl options based on a specified 
>>> profile.
>>> This userspace package has stopped working > 5.13 due to the following
>>> commit:
>>>
>>> 8a99b6833c88 "(sched: Move SCHED_DEBUG sysctl to debugfs)"
>>>
>>> This commit moved some important real-time sysctl knobs to debugfs in
>>> 5.13-rc1.  It also appears some of the sysctl options were not moved,
>>> sched_min_granularity_ns, for example.
>>>
>>> I was hoping to get some feedback on how to approach this. Would 
>>> upstream
>>> real-time consider accepting a patch to the 5.15 real-time patch set 
>>> that
>>> reverts this commit?  Or a new patch that adds the sysctl settings 
>>> back?
>>> Any other ideas or feedback would be appreciated!
>> None of those knobs were available when SCHED_DEBUG=n, so relying on
>> them is your error to begin with.
> We have had SCHED_DEBUG=y while this kernel is in beta, so you are 
> correct.  I need to investigate a different approach.
>>
>> Secondly, real-time? Which if those values affects anything in
>> SCHED_FIFO/RR/DEADLINE ?
> I am in the process of understanding how tuned[0] works. Many users 
> have reported success using tuned.  One use case for tuned is to 
> assign isolated cores to real-time processes and then move managed 
> IRQs out of these isolated cores.  This can be done easily with tuned 
> (I will research if there are other options to suggest). However, 
> tuned is trying to set the affected values when enabling a profile, 
> such as the real-time profile (Tuned offers many profiles based on a 
> workload type).
>
> I will investigate further to answer your point of what in 
> SCHED_FIFO/RR/DEADLINE those values affect.  It could be those values 
> are not needed at all. The dependency on them might be left over from 
> some need that no longer exists.
>
> Thank you again for the feedback!
>
> [0] https://tuned-project.org/
>
I also see that the github repo[0] is at version 2.19-rc1, and the 
version being testing is 2.15.  Many of the issues we see may already be 
resolved.

I will find out how I can participate in the tuned community.  At a 
minimum, I can start with testing and bug fixes.

Thanks again for your feedback, Peter.  And sorry for the noise.

Thanks,

Joe

[0] https://github.com/redhat-performance/tuned