linux-kernel - Re: [PATCH v2 3/2] sched/deadline: Check bandwidth overflow earlier for hotplug

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Message-ID: <Z630nGN1IHhyYIYl@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 13 Feb 2025 14:33:16 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Christian Loehle <christian.loehle@....com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
	Jon Hunter <jonathanh@...dia.com>,
	Thierry Reding <treding@...dia.com>,
	Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutny <mkoutny@...e.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Phil Auld <pauld@...hat.com>, Qais Yousef <qyousef@...alina.io>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Joel Fernandes (Google)" <joel@...lfernandes.org>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Aashish Sharma <shraash@...gle.com>,
	Shin Kawamura <kawasin@...gle.com>,
	Vineeth Remanan Pillai <vineeth@...byteword.org>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2 3/2] sched/deadline: Check bandwidth overflow earlier
 for hotplug

On 13/02/25 12:27, Christian Loehle wrote:
> On 2/13/25 06:20, Juri Lelli wrote:
> > On 12/02/25 19:22, Dietmar Eggemann wrote:
> >> On 11/02/2025 11:42, Juri Lelli wrote:
> > 
> > ...
> > 
> >>> What about we actually ignore them consistently? We already do that for
> >>> admission control, so maybe we can do that when rebuilding domains as
> >>> well (until we find maybe a better way to deal with them).
> >>>
> >>> Does the following make any difference?
> >>
> >> It at least seems to solve the issue. And like you mentioned on irc, we
> >> don't know the bw req of sugov anyway.
> >>
> >> So with this change we start with 'dl_bw->total_bw = 0' even w/ sugov tasks.
> >>
> >> dl_rq[0]:
> >>   .dl_nr_running                 : 0
> >>   .dl_bw->bw                     : 996147
> >>   .dl_bw->total_bw               : 0       <-- !
> >>
> >> IMHO, people who want to run serious DL can always check whether there
> >> are already these infrastructural DL tasks or even avoid schedutil.
> > 
> > It definitely not ideal and admittedly gross, but not worse than what we
> > are doing already considering we ignore sugovs at AC and the current
> > bandwidth allocation its there only to help with PI. So, duck tape. :/
> > 
> > A more proper way to work with this would entail coming up with sensible
> > bandwidth allocation for sugovs, but that's most probably hardware
> > specific, so I am not sure how we can make that general enough.
> > 
> > Anyway, looks like Jon was still seeing the issue. I asked him to verify
> > he is using all the proposed changes. Let's see what he reports.
> 
> FWIW it also fixes my reproducer.
> 
> I agree that dummy numbers for sugov bw is futile, but real bw numbers
> also don't make a lot of sense (what if we exceed them? The system
> won't be able to change frequency, i.e. might not be able to provide
> bw for other DL tasks then either?).
> I'm slightly worried about now allowing the last legal CPU for a sugov
> cluster to offline, which would lead to a cluster still being active
> but sugov DL unable to run anywhere. I can't reproduce this currently
> though. Is this an issue in theory? Or am I missing something?

Not sure I get what your worry is, sorry. In my understanding when the
last cpu of a policy/cluster gets offlined the corresponding sugov
kthread gets stopped as well (sugov_exit)?