linux-kernel - Re: [PATCH v2 00/10] Add kernel cmdline option for rt_group

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Message-ID: <s2omlhmorntg4bwjkmtbxhadeqfo667pbowzskdzbk3yxqdbfw@nvvw5bff6imc>
Date: Thu, 3 Apr 2025 14:17:02 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
	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>, Frederic Weisbecker <fweisbecker@...e.com>
Subject: Re: [PATCH v2 00/10] Add kernel cmdline option for rt_group_sched

On Tue, Apr 01, 2025 at 01:05:08PM +0200, Peter Zijlstra <peterz@...radead.org> wrote:
> > By default RT groups are available as originally but the user can
> > pass rt_group_sched=0 kernel cmdline parameter that disables the
> > grouping and behavior is like with !CONFIG_RT_GROUP_SCHED (with certain
> > runtime overhead).
> > 
> ...
> 
> Right, so at OSPM we had a proposal for a cgroup-v2 variant of all this
> that's based on deadline servers.

Interesting, are there any slides or recording available?

> And I am hoping we can eventually either fully deprecate the v1 thing
> or re-implement it sufficiently close without breaking the interface.

I converged to discourate rt_groups for these reasons:
1) They aren't RT guarantee for workloads
  - especially when it's possible to configure different periods
2) They aren't containment of RT tasks
  - RT task throttled in a group may hold a shared resource and thus its
    issues propagate to RT tasks in different groups
3) The allocation model [2] is difficult to configure
  - to honor delegation and reasonable default
  - illustration of another allocation model resource are cpuset cpus,
    whose abstraction in cgroup v2 is quite sophisticated

Based on that, I'm not proponent of any RT groups support in cgroup v2
(I'd need to see a use case where it could be justified). IIUC, the
deadline servers could help with 1).

> But this is purely about enabling cgroup-v1 usage, right?

Yes, users need to explicitly be on cgroup v1 (IOW they're stuck on v1
because of reliance on RT groups).

> You meantion some overhead of having this on, is that measured and in
> the patches?

I expect most would be affected RT task users who go from
!CONFIG_RT_GROUP_SCHED to CONFIG_RT_GROUP_SCHED and
CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED. That's my perception from code
that I touched but I haven't measured anything. Would this be
an interesting datum?

Thanks,
Michal

[1] https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#allocations