linux-kernel - Re: [PATCH 1/3] panic: Disable crash_kexec_post

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Message-ID: <20150714182336.GB3912@redhat.com>
Date:	Tue, 14 Jul 2015 14:23:36 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	dwalker@...o99.com, Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mips@...ux-mips.org, Baoquan He <bhe@...hat.com>,
	linux-sh@...r.kernel.org, linux-s390@...r.kernel.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-metag@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] panic: Disable crash_kexec_post_notifiers if kdump
 is not available

On Tue, Jul 14, 2015 at 01:01:12PM -0500, Eric W. Biederman wrote:
> Vivek Goyal <vgoyal@...hat.com> writes:
> 
> > On Tue, Jul 14, 2015 at 05:29:53PM +0000, dwalker@...o99.com wrote:
> >
> > [..]
> >> > >> > If a machine is failing, there are high chance it can't deliver you the
> >> > >> > notification. Detecting that failure suing some kind of polling mechanism
> >> > >> > might be more reliable. And it will make even kdump mechanism more
> >> > >> > reliable so that it does not have to run panic notifiers after the crash.
> >> > >> 
> >> > >> I think what your suggesting is that my company should change how it's hardware works
> >> > >> and that's not really an option for me. This isn't a simple thing like checking over the
> >> > >> network if the machine is down or not, this is way more complex hardware design.
> >> > >
> >> > > That means you are ready to live with an unreliable design. There might be
> >> > > cases where notifier does not get run properly and you will not do switch
> >> > > despite the fact that OS has failed. I was just trying to nudge you in
> >> > > a direction which could be more reliable mechanism.
> >> > 
> >> > Sigh I see some deep confusion going on here.
> >> > 
> >> > The panic notifiers are just that panic notifiers.  They have not been
> >> > nor should they be tied to kexec.   If those notifiers force a switch
> >> > over of between machines I fail to see why you would care if it was
> >> > kexec or another panic situation that is forcing that switchover.
> >> 
> >> Hidehiro isn't fixing the failover situation on my side, he's fixing register
> >> information collection when crash_kexec_post_notifiers is used.
> >
> > Sure. Given that we have created this new parameter, let us fix it so that
> > we can capture the other cpu register state in crash dump.
> >
> > I am little disappointed that it was not tested well when this parameter was
> > introuced. We should have atleast tested it to the extent to see if there
> > is proper cpu state present for all cpus in the crash dump.
> >
> > At that point of time it looked like a simple modification
> > to allow panic notifiers before crash_kexec().
> 
> Either that or we say no one cares enough, and it known broken so let's
> just revert the fool thing.

Masami, you introduced this option. Are you fine with the revert? Is it
really being used and tested?

> I honestly can't see how to support panic notifiers, before kexec.
> There is no way to tell what is being done and all of the pieces
> including smp_send_stop are known to be buggy.

we should be able to replace smp_send_stop() with what crash_kexec() is
doing to stop the machine? If yes, then it should be fine I guess. This
parameter description clearly says that specify it at your own risk. So
we are not issuing a big support statement for successful kdump after
panic notifiers. If it is something fixable, otherwise user needs
to deal with it.

Thanks
Vivek
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