linux-kernel - Re: [PATCH v3 4/8] sched/deadline: Rebuild root domain accounting after every update

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Message-ID: <Z9GWAbxuddrTzCS9@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 12 Mar 2025 15:11:13 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Waiman Long <llong@...hat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	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>, Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Qais Yousef <qyousef@...alina.io>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Swapnil Sapkal <swapnil.sapkal@....com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Phil Auld <pauld@...hat.com>, luca.abeni@...tannapisa.it,
	tommaso.cucinotta@...tannapisa.it,
	Jon Hunter <jonathanh@...dia.com>
Subject: Re: [PATCH v3 4/8] sched/deadline: Rebuild root domain accounting
 after every update

On 12/03/25 09:55, Waiman Long wrote:
> On 3/12/25 6:09 AM, Juri Lelli wrote:
> > On 12/03/25 10:53, Dietmar Eggemann wrote:
> > > On 11/03/2025 15:51, Waiman Long wrote:
> > ...
> > 
> > > > You are right. cpuhp_tasks_frozen will be set in the suspend/resume
> > > > case. In that case, we do need to add a cpuset helper to acquire the
> > > > cpuset_mutex. A test patch as follows (no testing done yet):
> > ...
> > 
> > > This seems to work.
> > Thanks for testing!
> > 
> > Waiman, how do you like to proceed. Separate patch (in this case can you
> > please send me that with changelog etc.) or incorporate your changes
> > into my original patch and possibly, if you like, add Co-authored-by?
> I think it will be better to merge into a single patch to avoid having a
> broken patch. It is up to you if you want me as a co-author. I don't really
> mind.
> > 
> > > But what about a !CONFIG_CPUSETS build. In this case we won't have
> > > this DL accounting update during suspend/resume since
> > > dl_rebuild_rd_accounting() is empty.
> > I unfortunately very much suspect !CPUSETS accounting is broken. But if
> > that is indeed the case, it has been broken for a while. :(
> Without CONFIG_CPUSETS, there will be one and only one global sched domain.
> Will this still be a problem?

Still need to double check. But I have a feeling we don't restore
accounting correctly (at all?!) without CPUSETS. Orthogonal to this
issue though, as if we don't, we didn't so far. :/

> > Will need to double check that, but I would probably do it later on
> > separated from this set that at least seems to cure the most common
> > cases. What do people think?
> 
> I am not aware of any distros without setting CONFIG_CPUSETS. So it is
> mostly a theoretical problem if there is one. So I would recommend going
> ahead with the current patch series instead of spending more time
> investigating this issue.

And I would agree (and then find time to look better into !CPUSETS
case). If nobody objects, I will refresh the series including Waiman's
changes and repost.

Thanks!
Juri