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

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Message-ID: <Z7NNHmGgrEF666W_@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 17 Feb 2025 15:52:14 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Qais Yousef <qyousef@...alina.io>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
	Christian Loehle <christian.loehle@....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>,
	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 16/02/25 16:33, Qais Yousef wrote:
> On 02/13/25 07:20, 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.
> 
> I haven't been following the problem closely, but one thing I was considering
> and I don't know if it makes sense to you and could help with this problem too.
> Shall we lump sugov with stopper class or create a new sched_class (seems
> unnecessary, I think stopper should do)? With the consolidate cpufreq update
> patch I've been working on Vincent raised issues with potential new ctx switch
> and to improve that I needed to look at improving sugov wakeup path. If we
> decouple it from DL I think that might fix your problem here and could allow us
> to special case it for other problems like the ones I faced more easily without
> missing up with DL.
> 
> Has the time come to consider retire the simple solution of making sugov a fake
> DL task?

Problem is that 'ideally' we would want to explicitly take sugovs into
account when designing the system. We don't do that currently as a
'temporary solution' that seemed simpler than a proper approach (started
wondering if it's indeed simpler). So, not sure if moving sugovs outside
DL is something we want to do.

Thanks,
Juri