Re: [PATCH] rt: cpufreq: Fix cpu hotplug hang

From: Sebastian Siewior <bigeasy@linutronix.de>
To: Ran Wang <ran.wang_1@nxp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Jiafei Pan <jiafei.pan@nxp.com>,
	linux-rt-users@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Viresh Kumar <viresh.kumar@linaro.org>
Subject: Re: [PATCH] rt: cpufreq: Fix cpu hotplug hang
Date: Mon, 22 Feb 2021 15:01:07 +0100	[thread overview]
Message-ID: <20210222140107.qk2ymlwyyxspluge@linutronix.de> (raw)
In-Reply-To: <20210219084420.23379-1-ran.wang_1@nxp.com>

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?

Sebastian