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From: Ran Wang <ran.wang_1@nxp.com>
To: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jiafei Pan <jiafei.pan@nxp.com>,
	"linux-rt-users@vger.kernel.org" <linux-rt-users@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: RE: [PATCH] rt: cpufreq: Fix cpu hotplug hang
Date: Thu, 25 Feb 2021 02:31:18 +0000
Message-ID: <AM6PR04MB5413EABA6643AA035183DDDEF19E9@AM6PR04MB5413.eurprd04.prod.outlook.com> (raw)
In-Reply-To: <20210224163745.5lqhnjviznsjojtl@linutronix.de>

Hi Sebastian,

On Thursday, February 25, 2021 12:38 AM, Sebastian Siewior wrote:
> 
> On 2021-02-23 08:43:08 [+0530], Viresh Kumar wrote:
> > On 23-02-21, 02:26, Ran Wang wrote:
> > > On Monday, February 22, 2021 10:01 PM, Sebastian Siewior wrote:
> > > > On 2021-02-19 16:44:20 [+0800], Ran Wang wrote:
> > > > > When selecting PREEMPT_RT, cpufreq_driver->stop_cpu(policy)
> > > > > might got stuck due to irq_work_sync() pending for work on
> > > > > lazy_list. That’s because lazy_list may have no chance to be
> > > > > served in softirq context sometimes. Below is one of scenarios that was captured:
> > > > >
> > > > > ...
> > > > > ret_from_fork
> > > > >  kthread
> > > > >   smpboot_thread_fn
> > > > >    cpuhp_thread_fun
> > > > >     cpuhp_invoke_callback: state: 193
> > > > >      cpuhp_cpufreq_online
> > > > >       cpufreq_online
> > > > >        cpufreq_driver->stop_cpu(policy);
> > > > >         cpufreq_dbs_governor_stop
> > > > >          sugov_stop  // kernel/sched/cpufreq_schedutil.c
> > > > >           irq_work_sync(&sg_policy->irq_work);
> > > > >
> > > > > This is observed on LX2160ARDB (16 A72 cores) with cpufreq
> > > > > governor of ‘schedutil’ or ‘ondemand’.
> > > >
> > > > While staring at it, why do we invoke schedule_work_on() and
> > > > kthread_queue_work() from inside irq_work() instead invoking it directly? It raises an interrupt in which it kicks a user thread.
> > > > Couldn't we do it without irq_work?
> >
> > Because we reach there from scheduler's context, which must be
> > hard-irq context..
> 
> Oha. Too bad.
> Ran, any chance to figure out why the softirq can not be served?
> ksoftirqd and the timer should be up.
> Any numbers on how often that is irq_work is scheduled?

Sure, let me add some trace events to collect more info.
 
Regards,
Ran

  reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19  8:44 Ran Wang
2021-02-22 14:01 ` Sebastian Siewior
2021-02-23  2:26   ` Ran Wang
2021-02-23  3:13     ` Viresh Kumar
2021-02-24 16:37       ` Sebastian Siewior
2021-02-25  2:31         ` Ran Wang [this message]
2021-03-23  7:00         ` Ran Wang

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