linux-kernel - Re: [RFC PATCH v3 00/24] Hierarchical Constant Bandwidth Server

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Message-ID: <aN4_KE-5kOCbpQux@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 2 Oct 2025 10:00:24 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Yuri Andriaccio <yurand2000@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org,
	Luca Abeni <luca.abeni@...tannapisa.it>,
	Yuri Andriaccio <yuri.andriaccio@...tannapisa.it>
Subject: Re: [RFC PATCH v3 00/24] Hierarchical Constant Bandwidth Server

Hello!

On 29/09/25 11:21, Yuri Andriaccio wrote:
> Hello,
> 
> This is the v3 for Hierarchical Constant Bandwidth Server, aiming at replacing
> the current RT_GROUP_SCHED mechanism with something more robust and
> theoretically sound. The patchset has been presented at OSPM25
> (https://retis.sssup.it/ospm-summit/), and a summary of its inner workings can
> be found at https://lwn.net/Articles/1021332/ . You can find the previous
> versions of this patchset at the bottom of the page, in particular version 1
> which talks in more detail what this patchset is all about and how it is
> implemented.
> 
> This v3 version further reworks some of the patches as suggested by Juri Lelli.
> While most of the work is refactorings, the following were also changed:
> - The first patch which removed fair-servers' bandwidth accounting has been
>   removed, as it was deemed wrong. You can find the last version of this removed
>   patch, just for history reasons, here:
>   https://lore.kernel.org/all/20250903114448.664452-1-yurand2000@gmail.com/

Peter wasn't indeed happy with that patch, but I am not sure we finished
that discussion. Both myself and Luca had further objections to what
Peter said, but not further replies after (which can very well be a sign
that he is still adamnt in saying no go away :). Peter?

https://lore.kernel.org/lkml/aLk9BNnFYZ3bhVAE@jlelli-thinkpadt14gen4.remote.csb/
https://lore.kernel.org/lkml/20250904091217.78de3dde@luca64/

...

> For this v3 version we've also polished the testing system we are using and made
> it public for testers to run on their own machines. The source code can be found
> at https://github.com/Yurand2000/HCBS-Test-Suite , along with a README that
> explains how to use it. Nonetheless I've reported a description of the tools and
> instruction later in the page.

Thanks for this. Quite cool. I tried to run the tests, but it looks like
the migration set brought my qemu 8 CPUs system to an halt (had to kill
the thing). I will try again.

I have been working a bit in trying to come up with a testing framework
for SCHED_DEADLINE, which I didn't unfortunately posted yet mostly
because I was waiting for the discussion about the patch mentioned above
to settle (which would require adaptation in the tests that check for
bandwidth limits). You can find the idea here [1]. It's thought to be
an addition to kselftests. I believe your test suite can be extended to
cover the tests I implemented and more, so I am not super attached to my
attempt, but it would be good (I think) to converge to something that we
can maintain for the future, so maybe have a plan to possibly merge the
suites. What do you think?

1 - https://github.com/jlelli/linux/commits/experimental/deadline-selftests-scripts/

...

> Testing v3:
> 
> The HCBS mechanism has been evaluated on several syntetic tests which are
> designed to stress the HCBS scheduler and verify that non-interference and
> mathematical schedulability guarantees are really enforced by the scheduling
> algorithm.

...

> The tasksets minimal requirements were computed using a closed-source software,
> explaining why the tasksets are supplied separately. A open-source analyser is
> being written to update this step in the future and also allow for more
> customization for the testers.

On this (generation of random taskset and corresponding schedulability
checks) I also started working on a different tool, rt-audit.

https://github.com/jlelli/rt-audit

It's very simple, the tool, the random generation and the checks. But I
honestly like the idea that it's based on rt-app. Please take a look if
you have a chance and tell me what you think. Again, I feel it would be
good to converge towards something open that we can maintain.

I will be trying to find time to continue testing and reviewing this RFC
of course.

Thanks,
Juri