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Date:   Sat, 17 Dec 2022 22:10:11 +0000
From:   Qais Yousef <qyousef@layalina.io>
To:     Waiman Long <longman@redhat.com>
Cc:     Juri Lelli <juri.lelli@redhat.com>, peterz@infradead.org,
        mingo@redhat.com, rostedt@goodmis.org, tj@kernel.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, lizefan@huawei.com,
        dietmar.eggemann@arm.com, 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>
Subject: Re: [PATCH v9 3/8] cpuset: Rebuild root domain deadline accounting
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Message-ID: <20221217221011.pqunktdfgkaoufrd@airbuntu>
References: <20190719140000.31694-1-juri.lelli@redhat.com>
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On 12/16/22 21:26, Waiman Long wrote:
> On 12/16/22 18:35, Qais Yousef wrote:
> > Hi
> > 
> > On 07/19/19 15:59, Juri Lelli wrote:
> > > When the topology of root domains is modified by CPUset or CPUhotplug
> > > operations information about the current deadline bandwidth held in the
> > > root domain is lost.
> > > 
> > > This patch addresses the issue by recalculating the lost deadline
> > > bandwidth information by circling through the deadline tasks held in
> > > CPUsets and adding their current load to the root domain they are
> > > associated with.
> > > 
> > > Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> > > Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> > > ---
> > We see that rebuild_root_domain() can take 10+ ms (I get a max of 20ms quite
> > consistently) on suspend/resume.
> > 
> > Do we actually need to rebuild_root_domain() if we're going through
> > a suspend/resume cycle?
> > 
> > ie: would something like the below make sense? We'd skip this logic if
> > cpuhp_tasks_frozen is set which indicates it's not a real hotplug operation but
> > we're suspending/resuming.
> > 
> > 
> > Cheers
> > 
> > --
> > Qais Yousef
> > 
> > 
> > --->8---
> > 
> > 
> >  From 4cfd50960ad872c5eb810ad3038eaf840bab5182 Mon Sep 17 00:00:00 2001
> > From: Qais Yousef <qyousef@layalina.io>
> > Date: Tue, 29 Nov 2022 19:01:52 +0000
> > Subject: [PATCH] sched: cpuset: Don't rebuild sched domains on suspend-resume
> > 
> > Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
> > enabled rebuilding sched domain on cpuset and hotplug operations to
> > correct deadline accounting.
> > 
> > Rebuilding sched domain is a slow operation and we see 10+ ms delays
> > on suspend-resume because of that.
> > 
> > Since nothing is expected to change on suspend-resume operation; skip
> > rebuilding the sched domains to regain some of the time lost.
> > 
> > Debugged-by: Rick Yiu <rickyiu@google.com>
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> >   kernel/cgroup/cpuset.c  | 6 ++++++
> >   kernel/sched/deadline.c | 3 +++
> >   2 files changed, 9 insertions(+)
> > 
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index b474289c15b8..2ff68d625b7b 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -1067,6 +1067,9 @@ static void update_tasks_root_domain(struct cpuset *cs)
> >   	struct css_task_iter it;
> >   	struct task_struct *task;
> > +	if (cpuhp_tasks_frozen)
> > +		return;
> > +
> >   	css_task_iter_start(&cs->css, 0, &it);
> >   	while ((task = css_task_iter_next(&it)))
> > @@ -1084,6 +1087,9 @@ static void rebuild_root_domains(void)
> >   	lockdep_assert_cpus_held();
> >   	lockdep_assert_held(&sched_domains_mutex);
> > +	if (cpuhp_tasks_frozen)
> > +		return;
> > +
> >   	rcu_read_lock();
> >   	/*
> 
> rebuild_root_domains() is the only caller of update_tasks_root_domain(). So
> the first hunk is redundant as update_tasks_root_domain() won't be called
> when cpuhp_tasks_frozen is set.

True. I went overzealous and protected all the functions.


Thanks!

--
Qais Yousef