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

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Date:	Mon, 29 Jun 2015 08:26:15 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>
Cc:	dwalker@...o99.com,
	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,
	ebiederm@...ssion.com, linux-kernel@...r.kernel.org
Subject: Re: kexec crash kernel running with watchdog enabled

On Mon, Jun 29, 2015 at 06:53:29PM +0900, Hidehiro Kawai wrote:
> Hi,
> 
> (2015/06/27 3:33), dwalker@...o99.com wrote:> On Fri, Jun 26, 2015 at 10:02:00AM +0900, Hidehiro Kawai wrote:
> >> Hi,
> >>
> >> (2015/06/25 1:31), dwalker@...o99.com wrote:
> >>> 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.
> >>
> >> No, that doesn't works.  Some notifiers assume that they run in
> >> single core mode.
> >>
> >> Another possible solution is to add notifiers just after
> >> machine_crash_shutdown() like this:
> >>
> >> void panic(const char *fmt, ...)
> >> ...
> >> -	if (!crash_kexec_post_notifiers)
> >> -		crash_kexec(NULL);
> >> +	crash_kexec(NULL, buf);
> >>
> >>   and
> >>
> >> -void crash_kexec(struct pt_regs *regs)
> >> +void crash_kexec(struct pt_regs *regs, char *msg)
> >>  ...
> >>  		if (kexec_crash_image) {
> >>  			struct pt_regs fixed_regs;
> >>  
> >>  			crash_setup_regs(&fixed_regs, regs);
> >>  			crash_save_vmcoreinfo();
> >>  			machine_crash_shutdown(&fixed_regs);
> >> +			if (crash_kexec_post_notifiers) {
> >> +				kmsg_dump(KMSG_DUMP_PANIC);
> >> +				atomic_notifier_call_chain(&panic_notifier_list, 0, msg);
> >> +			}
> >>                         machine_kexec(kexec_crash_image);
> >>
> >> Most of archs stop other cores in machine_crash_shutdown(),
> >> so it will work well.  Furthermore, it simplifies the special
> >> case where crash_kexec() is called without entering panic().
> >>
> >> However, we need some tweaks for sh and s390 cases.  As for sh,
> >> it seems not to stop other cores in the crash_kexec() sequence
> >> (kdump support is incompleted?).  For s390, smp_send_stop() is
> >> called in machine_kexec() but not machine_crash_shutdown().
> > 
> > You could add an ifdef into the __setup() to filter out s390 and sh, until we figure out what
> > to do there. So the "crash_kexec_post_notifiers" wouldn't be available for those platforms.
> 
> I agree on disabling the "crash_kexec_post_notifiers" feature for
> s390 and sh at this time.  Also, we should make this feature effective
> only if CONFIG_CRASH_DUMP=y.  Otherwise, it makes no sense.
> 
> I'll prepare the bug fix patch.  Please wait a moment.

Why to add a patch for that. Just inform the user that don't need
crash_kexec_post_notifiers on s390 and sh?

Thanks
Vivek
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