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

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Message-ID: <aPYDhjqe99F91FTW@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 20 Oct 2025 11:40:22 +0200
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

On 02/10/25 10:00, Juri Lelli wrote:
> 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/

I had a quick chat with Peter on IRC about this. We now seem to agree
that a third option would be to move to explicitly account dl-server(s),
correspondingly moving from a 95% to 100% limit. That would also make our
life easier in the future with additional dl-servers (e.g. scx-server).

What do you think?