Re: [ANNOUNCE] Merkey's Kernel Debugger

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Aug 08, 2008 at 09:50:00AM -0500, Cliff Wickman wrote:
> On Fri, Aug 08, 2008 at 09:29:53AM -0400, Vivek Goyal wrote:
> > On Fri, Aug 08, 2008 at 04:29:16AM +0200, Andi Kleen wrote:
> > > > panic() is the only place where kdump gets a chance to run first and
> > > > panic notifiers are not executed.
> > > 
> > > To be fully clear panic() that is called outside oops/exception context
> > > 
> > > s/panic/die notifiers/
> > > 
> > > > 
> > > > To me so far only in kernel debugger seems to be a reasonable candiate
> > > 
> > > Yes a kernel debugger should be able to hook into panic()
> > > 
> > > In fact it can do that already by just setting a break point,
> > > but clearly having a real notifier is preferable.
> > > 
> > > The use case would be then that the kernel debugger would
> > > have some command to trigger a dump.
> > > 
> > > > which needs to run before kdump after a panic event. If a debugger
> > > > is really getting merged into the kernel, then I think debugger can
> > > 
> > > kgdb is already merged. Also the x86 notifiers are general
> > > enough that there are a couple of debuggers floating around
> > > that are just using existing interfaces (as in need very little in terms
> > > of core patching) 
> > > 
> > > > put a hook in the panic() before kdump. Wouldn't this solve the problem?
> > > 
> > > Yes it would, but right now there is no such hook. Also if there 
> > > was such a hook kdump could use it like everyone else.  
> > > 
> > > There's a priority scheme in notifiers so you can still run usually last.
> > 
> > Hi Andi,
> > 
> > IIUC, there are two lists for exception and panic notifications. All the
> > exceptios, NMI related notifications go through "die_chain" and
> > all the panic notifications are done through "panic_notifier_list".
> > 
> > Are you suggesting that kdump should be put onto panic_notifier_list, in
> > such a way so that it runs last?
> > 
> > Just few points to ponder.
> > 
> > - panic_notifier_list is exported and any module can register and make use
> >   of it. As you mentioned in your other mail, there are lot of drivers out
> >   there with crappy code and if we do it, all the drivers get a chance
> >   to do stuff after panic() and there is no gurantee that kdump code will
> >   ever get a chance to run.
> > 
> > - Kdump is built on the philosophy that after a panic(), one should do as
> >   as little as possible in the kernel and all the actions should be
> >   deferred to new kernel. That's why we recommend that all the panic
> >   notifier actions (except debugger), should be done in second kernel. It
> >   does introduce a little delay in notification but it also makes it more
> >   reliable.
> > 
> > - Neil Horman, has already provided infrastructure so that one can put
> >   it user space code in second kernel's initrd and it will be executed. 
> >   This can be easily done for modules also. 
> > 
> > But somehow nobody seems to be interested in doing things in second kernel
> > and everybody wants to run its post panic code in the first kernel. So
> > far, except debugger, we have not run into any strong case which needs to
> > run post panic code in first kernel and things will not work out if post
> > panic actions are taken in second kernel.
> 
> In the case of the cross-partition driver, running panic notification in the
> second kernel is an interesting idea.
> 
> I discussed it with Robin Holt, who is more knowledgable than I on the
> details of that driver, and he told me that there is a great deal of
> state information needed for the notification.  It's easy to do in the
> first kernel, but extremely difficult in a second kernel.
> 

Generally what kind of state information has to be passed?

> Couldn't we have some tunable flexability in that area, to determine
> should run on a panic, and in what order?

May be that's the way forward. Export the list of registered handlers on
panic_notifier_list through sysfs or debugfs and also provide flexibility
that user can change the priorities from userspace. That should work
for all. 

Thanks
Vivek