Re: [PATCH] netpoll can lock up on low memory.

Andi Kleen
2005-08-06T11:29:24Z

On Sat, Aug 06, 2005 at 09:45:03AM +0200, Ingo Molnar wrote:
>
> * Andi Kleen <ak@suse.de> wrote:
>
> > On Fri, Aug 05, 2005 at 01:01:57PM -0700, Matt Mackall wrote:
> > > The netpoll philosophy is to assume that its traffic is an absolute
> > > priority - it is better to potentially hang trying to deliver a panic
> > > message than to give up and crash silently.
> >
> > That would be ok if netpoll was only used to deliver panics. But it is
> > not. It delivers all messages, and you cannot hang the kernel during
> > that. Actually even for panics it is wrong, because often it is more
> > important to reboot in a panic than (with a panic timeout) to actually
> > deliver the panic. That's needed e.g. in a failover cluster.
>
> without going into the merits of this discussion, reliable failover
> clusters must include (and do include) an external ability to cut power.
> No amount of in-kernel logic will prevent the kernel from hanging, given
> a bad enough kernel bug.

Ok, true, but we should do a best effort.

>
> So the right question is not 'can we prevent the kernel from hanging,
> ever' (we cannot), but 'which change makes it less likely for the kernel
> to hang'. (and, obviously: assuming all other kernel components are
> functioning per specification, netpoll itself most not hang :-)
>
> even a plain printk to VGA can hang in certain kernel crashes. Netpoll
> is more complex and thus has more exposure to hangs. E.g. netpoll relies
> on the network driver to correctly recycle skbs within a bound amount of
> time. If the network driver leaks skbs, it's game over for netpoll.

I don't think we even need to think about such rare cases,
until the easy cases ("everything hangs when the cable is pulled")
are not fixed.

> [ i'd prefer a hang over nondeterministic behavior, and e.g. losing
> console messages is sure nondeterministic behavior. What if the
> console message is "WARNING: the box has just been broken into"? ]

That just makes netconsole useless in production. If it causes frequenet
hangs people will not use it.

>
> we could do one thing (see the patch below): i think it would be useful
> to fill up the netlogging skb queue straight at initialization time.
> Especially if netpoll is used for dumping alone, the system might not be
> in a situation to fill up the queue at the point of crash, so better be
> a bit more prepared and keep the pipeline filled.

You're solving a completely different issue here?

-Andi