linux-kernel - Re: [RFC PATCH 0/3] sched/deadline: cpuset: Rework DEADLINE bandwidth restoration

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Date:   Wed, 15 Mar 2023 17:10:54 +0000
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Hao Luo <haoluo@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org,
        cgroups@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Wei Wang <wvw@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [RFC PATCH 0/3] sched/deadline: cpuset: Rework DEADLINE
 bandwidth restoration

On 15/03/23 14:55, Qais Yousef wrote:
> On 03/15/23 12:18, Juri Lelli wrote:
> > Qais reported [1] that iterating over all tasks when rebuilding root
> > domains for finding out which ones are DEADLINE and need their bandwidth
> > correctly restored on such root domains can be a costly operation (10+
> > ms delays on suspend-resume). He proposed we skip rebuilding root
> > domains for certain operations, but that approach seemed arch specific
> > and possibly prone to errors, as paths that ultimately trigger a rebuild
> > might be quite convoluted (thanks Qais for spending time on this!).
> 
> Thanks a lot for this! And sorry I couldn't provide something better.

Ah, no worries. Actually still have to convice myself what I have it's
actually better. :)

> > 
> > To fix the problem I instead would propose we
> > 
> >  1 - Bring back cpuset_mutex (so that we have write access to cpusets
> >      from scheduler operations - and we also fix some problems
> >      associated to percpu_cpuset_rwsem)
> >  2 - Keep track of the number of DEADLINE tasks belonging to each cpuset
> >  3 - Use this information to only perform the costly iteration if
> >      DEADLINE tasks are actually present in the cpuset for which a
> >      corresponding root domain is being rebuilt
> 
> nit:
> 
> Would you consider adding another patch to rename the functions?
> rebuild_root_domains() and update_tasks_root_domain() are deadline accounting
> specific functions and don't actually rebuild root domains.

Yep, can do.

Thanks,
Juri