linux-kernel - Re: kexec crash kernel running with watchdog enabled

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Message-ID: <20150625201626.GA24392@fifo99.com>
Date:	Thu, 25 Jun 2015 20:16:26 +0000
From:	dwalker@...o99.com
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
	david.daney@...ium.com, d.hatayama@...fujitsu.com,
	vgoyal@...hat.com, hidehiro.kawai.ez@...achi.com,
	linux-kernel@...r.kernel.org
Subject: Re: kexec crash kernel running with watchdog enabled

On Wed, Jun 24, 2015 at 12:06:03PM -0500, Eric W. Biederman wrote:
> dwalker@...o99.com writes:
> 
> > On Wed, Jun 24, 2015 at 03:52:48PM +0900, Masami Hiramatsu wrote:
> >> Hi,
> >> 
> >> On 2015/06/23 23:05, dwalker@...o99.com wrote:
> >> > 
> >> > Hi,
> >> > 
> >> > There was a commit in kernel/panic.c which altered when the kexec crash kernel is executed,
> >> > 
> >> > commit f06e5153f4ae2e2f3b0300f0e260e40cb7fefd45
> >> > Author: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> >> > Date:   Fri Jun 6 14:37:07 2014 -0700
> >> > 
> >> >     kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after panic_notifers
> >> > 
> >> > 
> >> > This made it possible for smp_send_stop() to be executed prior to calling the kexec crash
> >> > kernel.
> >> > 
> >> > The issue is that smp_send_stop() offlines the cores, and other code depend on the cores being online.
> >> > 
> >> > In my case on Octeon here's an example,
> >> > 
> >> > panic()
> >> >  crash_kexec()
> >> >   machine_crash_shutdown()
> >> >    octeon_generic_shutdown()
> >> > 
> >> > Inside octeon_generic_shutdown() the Octeon watchdog is shutdown for_each_online_cpu(), but since
> >> > most of the cpu's already got offlined in smp_send_stop() it means the watchdog is still alive on
> >> > those cores. This results in a reboot during the crash kernel execution.
> >> 
> >> Ah, I see.
> >> 
> >> > Another example seem to be in default_machine_crash_shutdown() where crash_kexec_prepare_cpus() depends
> >> > on an IPI for saving the registers on different cores. However, the cpu's are all offlined with
> >> > interrupts disabled so they won't be running those IPI's in this case.
> >> > 
> >> > I'm looking for any advice on how this should be fixed, or if it's already fixed. I'm not going to be
> >> > submitting a patch so if anyone wants to submit one feel free to do so.
> >> 
> >> Hmm, IMHO, when the cpu goes to offline in appropriate way(smp_send_stop), it should stop
> >> watchdog timer on the offlined cpu too.
> >> Or, you can also register crash handler which stops all watchdogs, but it's a bit tricky.
> >> 
> >
> > That doesn't really fix all the issue tho. As I was explaining generic MIPS code depends on the cpu's
> > effectively being online for crash data collection (with an IPI). This issue may effect other architectures also,
> > because smp_send_stop() offlines the cpu on other architectures also. I haven't surveyed the other architectures
> > enough to know what issue could happen from this tho.
> >
> > Is it possible to move the smp_send_stop() below the notifiers ? I'm
> > just throwing out ideas.
> 
> In general if you want reliability don't pass the kernel command line to
> run the panic notifiers.


We use the panic notifiers to "fail over" , or switch to a working machine. We could do
that in the crash kernel, but we would have to wait several seconds which is too long. Do
you know of a more stable way to accomplish that ?

Daniel
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