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

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Message-ID: <Z9Alq55RpuFqWT--@jlelli-thinkpadt14gen4.remote.csb>
Date: Tue, 11 Mar 2025 12:59:39 +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 10/03/25 20:16, Waiman Long wrote:
> On 3/10/25 3:18 PM, Waiman Long wrote:
> > 
> > On 3/10/25 2:54 PM, Dietmar Eggemann wrote:
> > > On 10/03/2025 10:37, Juri Lelli wrote:
> > > > Rebuilding of root domains accounting information (total_bw) is
> > > > currently broken on some cases, e.g. suspend/resume on aarch64. Problem
> > > Nit: Couldn't spot any arch dependency here. I guess it was just tested
> > > on Arm64 platforms so far.
> > > 
> > > [...]
> > > 
> > > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > > > index 44093339761c..363ad268a25b 100644
> > > > --- a/kernel/sched/topology.c
> > > > +++ b/kernel/sched/topology.c
> > > > @@ -2791,6 +2791,7 @@ void partition_sched_domains_locked(int
> > > > ndoms_new, cpumask_var_t doms_new[],
> > > >       ndoms_cur = ndoms_new;
> > > >         update_sched_domain_debugfs();
> > > > +    dl_rebuild_rd_accounting();
> > > Won't dl_rebuild_rd_accounting()'s lockdep_assert_held(&cpuset_mutex)
> > > barf when called via cpuhp's:
> > > 
> > > sched_cpu_deactivate()
> > > 
> > >    cpuset_cpu_inactive()
> > > 
> > >      partition_sched_domains()
> > > 
> > >        partition_sched_domains_locked()
> > > 
> > >          dl_rebuild_rd_accounting()
> > > 
> > > ?

Good catch. Guess I didn't notice while testing with LOCKDEP as I was
never able to hit this call path on my systems.

> > Right. If cpuhp_tasks_frozen is true, partition_sched_domains() will be
> > called without holding cpuset mutex.
> > 
> > Well, I think we will need an additional wrapper in cpuset.c that
> > acquires the cpuset_mutex first before calling partition_sched_domains()
> > and use the new wrapper in these cases.
> 
> Actually, partition_sched_domains() is called with the special arguments (1,
> NULL, NULL) to reset the domain to a single one. So perhaps something like
> the following will be enough to avoid this problem.

I think this would work, as we will still rebuild the accounting after
last CPU comes back from suspend. The thing I am still not sure about is
what we want to do in case we have DEADLINE tasks around, since with
this I belive we would be ignoring them and let suspend proceed.