Date: Thu, 14 Nov 2019 20:10:20 +0200 From: Daniel Braniss <danny@cs.huji.ac.il> To: Ian Lepore <ian@freebsd.org> Cc: freebsd-hackers <freebsd-hackers@freebsd.org> Subject: Re: can the hardware watchdog reboot a hung kernel? Message-ID: <BEC1714A-2361-4B62-BEB9-82808920C269@cs.huji.ac.il> In-Reply-To: <828605fef472e04311c83a7de0d1f4df429ae717.camel@freebsd.org> References: <EC4DB495-55D0-44BB-8D6A-0301785FADC7@cs.huji.ac.il> <9cded04a-9ae1-881e-3962-7ef0322e96ed@grosbein.net> <2AD912BF-97B0-421D-B561-722D74864DC9@cs.huji.ac.il> <828605fef472e04311c83a7de0d1f4df429ae717.camel@freebsd.org>
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> On 14 Nov 2019, at 18:02, Ian Lepore <ian@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@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 :-) >
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