Re: Using debug-key 'o: Dump IOMMU p2m table, locks up machine

[Keir Fraser <keir.xen@gmail.com>]

On 04/09/2012 09:04, "Keir Fraser" <keir.xen@gmail.com> wrote:

> On 04/09/2012 08:55, "Jan Beulich" <JBeulich@suse.com> wrote:
> 
>>> Jan, Yes you could be right, if Sander is pinning CPUs. Anyway, I wasn't
>>> going to expend too much brain power on this situation. The case of spending
>>> a few minutes in one key handler is not one I think is particularly sane.
>> 
>> Which imo would call for reverting the patch. But then again, other
>> key handlers can easily take pretty long too (particularly on large
>> systems, albeit it is clear that the one here is particularly bad), and
>> declaring all of them pretty much useless probably isn't the best
>> choice (as then we could as well rip them all out).
>> 
>> Bottom line - _I_ think we should try to do something about this.
>> An apparent option would be to have low priority tasklets (for
>> just this purpose, as all others we certainly want to take priority),
>> if that can reasonably be integrated with the schedulers.
> 
> Do you expect to be able to use the log-running key handlers and still need
> a running system afterwards (rather than using them as a final
> dump-everything when the system has already gone bad)? Then I suppose you
> would need something like this, with voluntary preemption in the key
> handlers. You then need to be able to recommence the keyhandlers where they
> left off, retaking locks, finding their place in lists, trees, etc, even
> when state of the system has significantly changed between preemption and
> resumption. Well, I'm sure it can be done, but can anyone be bothered.

My pragmatic take would be that: (a) Really long-running handlers that want
to dump every page mapping of every domain are pretty bloody stupid, and yes
we should consider if they are worthwhile at all; (b) moderately
long-running but useful handlers which nonetheless take a long time to dump
to Xen's console, I would consider a sysctl to allow dom0 to request dump
into a supplied memory buffer.

>  -- Keir
> 
>