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

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Message-ID: <20230315145514.vjoypwadprvpgwam@airbuntu>
Date:   Wed, 15 Mar 2023 14:55:14 +0000
From:   Qais Yousef <qyousef@...alina.io>
To:     Juri Lelli <juri.lelli@...hat.com>
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 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.

> 
> 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.


Thanks!

--
Qais Yousef

> 
> This set is also available from
> 
> https://github.com/jlelli/linux.git deadline/rework-cpusets
> 
> Feedback is more than welcome.
> 
> Best,
> Juri
> 
> 1 - https://lore.kernel.org/lkml/20230206221428.2125324-1-qyousef@layalina.io/
> 
> Juri Lelli (3):
>   sched/cpuset: Bring back cpuset_mutex
>   sched/cpuset: Keep track of SCHED_DEADLINE task in cpusets
>   cgroup/cpuset: Iterate only if DEADLINE tasks are present
> 
>  include/linux/cpuset.h |  12 ++-
>  kernel/cgroup/cgroup.c |   4 +
>  kernel/cgroup/cpuset.c | 175 +++++++++++++++++++++++------------------
>  kernel/sched/core.c    |  32 ++++++--
>  4 files changed, 137 insertions(+), 86 deletions(-)
> 
> -- 
> 2.39.2
>