From:   Juri Lelli <juri.lelli@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Qais Yousef <qyousef@layalina.io>,
        Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Hao Luo <haoluo@google.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-kernel@vger.kernel.org, luca.abeni@santannapisa.it,
        claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it,
        bristot@redhat.com, mathieu.poirier@linaro.org,
        cgroups@vger.kernel.org,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Wei Wang <wvw@google.com>, Rick Yiu <rickyiu@google.com>,
        Quentin Perret <qperret@google.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>
Subject: [RFC PATCH 0/3] sched/deadline: cpuset: Rework DEADLINE bandwidth restoration
Date:   Wed, 15 Mar 2023 12:18:09 +0000
Message-Id: <20230315121812.206079-1-juri.lelli@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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!).

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

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