can the hardware watchdog reboot a hung kernel?

[Daniel Braniss]

> On 14 Nov 2019, at 18:02, Ian Lepore <ian at freebsd.org> wrote:
> 
> On Thu, 2019-11-14 at 17:35 +0200, Daniel Braniss wrote:
>>> On 14 Nov 2019, at 17:28, Eugene Grosbein <eugen at grosbein.net>
>>> wrote:
>>> 
>>> 14.11.2019 21:52, Daniel Braniss wrote:
>>> 
>>>> hi,
>>>> I have serveral hundred Nano-pi NEO running, and sometimes they
>>>> hang, since there is no console
>>>> available, the only solution is to do a power cycle - not so easy
>>>> since they are distributed in three buildings :-)
>>>> 
>>>> I am looking at the watchdog stuff, but it seems that what I want
>>>> is not supported, i.e.
>>>> 	reboot the kernel when hung 
>>>> 
>>>> wishful thinking?
>>> 
>>> It's possible if the hardware has such a watchdog and kernel
>>> subsystem watchdog(4) supports it.
>>> rc.conf(5) manual page describes watchdogd_enable option.
>>> 
>> 
>> yes, but it relys  on user land, what if the kernel is hung? 
>> 
> 
> It relies on the userland daemon to issue the ioctl() calls to pet the
> dog.  If the kernel is hung, then userland code isn't going to run
> either, and the watchdog petting won't happen, and eventually the
> hardware reboots.
> 
> We use this at $work specifically to reboot if the kernel hangs, using
> this config:
> 
> watchdogd_enable=YES
> watchdogd_flags="-s 16 -t 64 -x 64"
> 
> That says the daemon should pet the dog every 16 seconds, and the
> hardware is programmed to reboot if 64 seconds elapses without petting.
> In addition, when watchdogd is shutdown normally (like during a normal
> system reboot) it doesn't disable the watchdog hardware, it sets the
> timeout to 64s to protect against any kind of hang during the reboot. 
> The -t and -x times can be different, 64s just happens to work well for
> us in both cases.
> 
> -- Ian
> 

ok, that is very encouraging, now a last question
how can i hang the kernel to test that the watchdog kicks in? apart from writing a kernel module :-)
 
>