linux-kernel - Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu hotplug when using schedutil

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Message-ID: <20220711171130.6390600b@gandalf.local.home>
Date:   Mon, 11 Jul 2022 17:11:30 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Qais Yousef <qais.yousef@....com>,
        Xuewen Yan <xuewen.yan@...soc.com>, rafael@...nel.org,
        viresh.kumar@...aro.org, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        ke.wang@...soc.com, xuewyan@...mail.com, linux-pm@...r.kernel.org,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu
 hotplug when using schedutil

On Mon, 11 Jul 2022 10:58:28 -1000
Tejun Heo <tj@...nel.org> wrote:

> I don't think lockdep would be able to track CPU1 -> CPU2 dependency here
> unfortunately.
> 
> > AFAIU:
> > 
> > 
> > CPU0                                     CPU1                                   CPU2
> > 
> > // attach task to a different
> > // cpuset cgroup via sysfs
> > __acquire(cgroup_threadgroup_rwsem)
> > 
> >                                          // pring up CPU2 online
> >                                          __acquire(cpu_hotplug_lock)
> >                                          // wait for CPU2 to come online

Should there be some annotation here that tells lockdep that CPU1 is now
blocked on CPU2?

Then this case would be caught by lockdep.

-- Steve


> >                                                                                 // bringup cpu online
> >                                                                                 // call cpufreq_online() which tries to create sugov kthread
> > __acquire(cpu_hotplug_lock)                                                     copy_process()
> >                                                                                    cgroup_can_fork()
> >                                                                                       cgroup_css_set_fork()
> >                                                                                       __acquire(cgroup_threadgroup_rwsem)
> > // blocks forever                        // blocks forever                            // blocks forever
> >