Re: Enabling balloon(4) in Xen dom0/domU kernels?

[Jean-Yves Migeon <jeanyves.migeon%free.fr@localhost>]

On 26.04.2011 23:12, Hugo Silva wrote:
> Had it briefly enabled with a 5.99.49 box, but with ballooning enabled
> it rebooted on its own 2 or 3 times.
> 
> I think the culprit might be swap on cgd though (or an interaction with
> the ballooning code? I have no idea).

Do you have ddb.onpanic set to 1?

> On several 5.1 domUs with very low memory (16-32M) I notice a hang where
> the domU will be consuming 100% CPU (xm top from dom0) and xm console
> seems to work but hangs right away, and ssh stops working too.

Balloon has not been pulled up to 5.1. What code did you take? The "old"
one with the BALLOON option in config file, or the recent one (committed
about a week ago)?

> Notably, to be able to compile a kernel on these VMs, I had to disable
> swap on cgd.
> 
> I'm not 100% sure it's 2 different problems, since the domU hangs at
> 100% and the dom0 just reboots. I have the dom0 running with swap on cgd
> and 768M mem for a week now, no crashes (it did crash with ballooning).
> There is that.
> 
> It's all a bit vague I know, but maybe someone else steps up with the
> same problem and we might be able to track it down..
> 
> Hugo
> 
> Wish I had more precise information, I know it's a bit vague.

Turn on ddb.onpanic, hopefully we'll get a bit more details on the context.

Note that balloon(4) cannot be triggered unless explicitly signaled,
either by starting a new domU (in which case, the Xentools will try to
get back memory to start the domain), or via the xm mem-set command.
Rest of the time, the associated LWP will remain idle and should not
modify memory reservation, both for dom0 or domUs.

I'll try reproducing this with swap on cgd. Is only swap backed by
cgd(4), or other file systems too (domain images, specific partitions, ...?)

-- 
Jean-Yves Migeon
jeanyves.migeon%free.fr@localhost