Re: [PATCH v2] reboot: Backup orderly_poweroff

From: Grygorii Strashko <grygorii.strashko@ti.com>
To: Ingo Molnar <mingo@kernel.org>,
	One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Keerthy <a0393675@ti.com>, Keerthy <j-keerthy@ti.com>,
	<linux-kernel@vger.kernel.org>, <edubezval@gmail.com>,
	<nm@ti.com>, <linux-pm@vger.kernel.org>,
	<linux-omap@vger.kernel.org>, <joel@jms.id.au>,
	<akpm@linux-foundation.org>,
	<linux-arm-kernel@lists.infradead.org>, <peterz@infradead.org>,
	<dyoung@redhat.com>, <josh@joshtriplett.org>,
	<mpe@ellerman.id.au>, Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH v2] reboot: Backup orderly_poweroff
Date: Fri, 15 Jan 2016 15:29:04 +0200	[thread overview]
Message-ID: <5698F420.2010500@ti.com> (raw)
In-Reply-To: <20160115101459.GB23349@gmail.com>

On 01/15/2016 12:14 PM, Ingo Molnar wrote:
> 
> * One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk> wrote:
> 
>>> If kernel_power_off() is called then the system should power off. No ifs and
>>> whens.
>>
>> Even if it doesn't the watchdog should kill it.
>>
>> That is broken on some platforms on the watchdog side as the
>> watchdog shuts down during our power off callbacks - because the system
>> firmware is too stupid to reset the watchdog as it powers back up (so
>> keeps rebooting).
>>
>> If you watchdog and firmware function properly you shouldn't even have to
>> care if you crash during the kernel power off.
> 
> That's a good point as well - if the system is 'stuck' for some notion of stuck,
> then watchdog drivers can help.
> 

Seems ARM doesn't have endless loop implemented in machine_power_off() - so,
not too much chances for Watchdog to fire.
void machine_power_off(void)
{
	local_irq_disable();
	smp_send_stop();

	if (pm_power_off)
		pm_power_off();

	--- endless loop ?
	--- or restart ?
}
[and even if it will be there - 20-30sec is usual timeout for Watchdog and this
enough time to burn the system in case of thermal emergency poweroff :(]

> Here it's unclear whether user-space even called the sys_reboot() system call.
> 

That's true - original log [1] has 
Nov 30 11:19:22 [    5.942769] thermal thermal_zone3: critical temperature reached(108 C),shutting down
[...]
Nov 30 11:19:24 [    7.387900] ahci 4a140000.sata: flags: 64bit ncq sntf stag pm led clo only pmp pio slum part ccc apst 
Nov 30 11:19:24 INIT: Switching to runlevel: 0
Nov 30 11:19:24 INIT: Sending processes the TERM signal

and there are no
[  220.004522] reboot: Power down

Also, It's not the first time this part of code is discussed (thermal emergency poweroff) [2],
so the good question, as for me, is it really required and safe to use orderly_poweroff() in
case of thermal emergency poweroff ([3] as example)?

In general, this kind of use case can be simulated using SysRq on any arch
- [3.290034] Freeing unused kernel memory: 492K (c0a67000 - c0ae2000)
  INIT: version 2.88 booting
  Starting udev
^^ The issue most probably might happens when system in the process of loading modules
So, once modules loading process is started - fire Sysrq "poweroff(o)"

[1] http://pastebin.ubuntu.com/14326688/
[2] https://lkml.org/lkml/2012/9/18/577
[3] http://review.omapzoom.org/#/c/34898/
-- 
regards,
-grygorii