linux-kernel - Re: [RFC PATCH v2 00/25] Hierarchical Constant Bandwidth Server

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Message-ID: <aJyb143HTlGqCBeo@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 13 Aug 2025 16:06:15 +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 v2 00/25]  Hierarchical Constant Bandwidth Server

Hi,

On 31/07/25 12:55, Yuri Andriaccio wrote:
> Hello,
> 
> This is the v2 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 v1 of this
> patchset at the bottom of the page, which talks in more detail what this
> patchset is all about and how it is implemented.
> 
> The big update for this v2 version is the addition of migration code, which
> allows to migrate tasks between different CPUs (following of course affinity
> settings).
> 
> As requested, we've split the big patches in smaller chunks in order to improve
> in readability. Additionally, it has been rebased on the latest tip/master to
> keep up with the latest scheduler updates and new features of dl_servers.
> 
> Last but not least, the first patch, which has been presented separately at
> https://lore.kernel.org/all/20250725164412.35912-1-yurand2000@gmail.com/ , is
> necessary to fully utilize the deadline bandwidth while keeping the fair-servers
> active. You can refer to the aforementioned link for details. The issue
> presented in this patch also reflects in HCBS: in the current version of the
> kernel, by default, 5% of the realtime bandwidth is reserved for fair-servers,
> 5% is not usable, and only the remaining 90% could be used by deadline tasks, or
> in our case, by HCBS dl_servers. The first patch addresses this issue and allows
> to fully utilize the default 95% of bandwidth for rt-tasks/servers.
> 
> Summary of the patches:
>      1) Account fair-servers bw separately from other dl tasks and servers bw.
>    2-5) Preparation patches, so that the RT classes' code can be used both
>         for normal and cgroup scheduling.
>   6-15) Implementation of HCBS, no migration and only one level hierarchy.
>         The old RT_GROUP_SCHED code is removed.
>  16-18) Remove cgroups v1 in favour of v2.
>     19) Add support for deeper hierarchies.
>  20-25) Add support for tasks migration.
> 
> Updates from v1:
> - Rebase to tip/master.

Would you mind sharing the baseline sha this set applies to? It looks
like it doesn't apply cleanly anymore.

Thanks!
Juri