Xen watchdog?

Xen watchdog?

[Richard W.M. Jones]

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I noticed that a Xen watchdog implementation was proposed on this list a 
few months back:

http://lists.xensource.com/archives/html/xen-devel/2007-05/msg01159.html

Has this been replaced with something else?  If not then did anything 
come of this and/or are there any plans for a watchdog in a future 
hypervisor?

Rich.

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Re: Xen watchdog?

[Keir Fraser]

I took a brief look at it. It was kind of big, with no great justification.
I would have written a simple little user daemon for kicking the watchdog
directly, rather than add extra stuff in our Linux kernel. But perhaps the
latter is the 'proper' way? If so, the driver probably needs review on lkml.

Either way, I think the hypervisor interface should support multiple
entities per guest trying to set up a watchdog timer. Even if just so that
entities that lose the race get -EBUSY. This would make it safer to use
without a single controlling entity in the guest kernel.

Are you up for kicking these patches into shape, Mark?

 -- Keir

On 25/9/07 09:37, "Richard W.M. Jones" <rjones@redhat.com> wrote:

> I noticed that a Xen watchdog implementation was proposed on this list a
> few months back:
> 
> http://lists.xensource.com/archives/html/xen-devel/2007-05/msg01159.html
> 
> Has this been replaced with something else?  If not then did anything
> come of this and/or are there any plans for a watchdog in a future
> hypervisor?
> 
> Rich.

Re: Xen watchdog?

[Mark Williamson]

> I took a brief look at it. It was kind of big, with no great justification.
> I would have written a simple little user daemon for kicking the watchdog
> directly, rather than add extra stuff in our Linux kernel. But perhaps the
> latter is the 'proper' way? If so, the driver probably needs review on
> lkml.

It's the Linux Way - a userspace app wouldn't work, since it wouldn't be able 
to export the /dev/watchdog device node that HA infrastructure needs.

To be honest, IIRC, it seemed to me like most watchdog drivers in Linux could 
share more common code and then all be a decent bit smaller - something I 
could look at if the Linux folks are interested but wasn't on my todo list at 
the time.

A fair bit of the bulk in the rest of the code was in making the dom0 tools be 
a bit more aware of the watchdog state.  This was needed to support features 
such as the "watchdog initiated reboot" flag, but isn't strictly needed for 
basic functionality.  I can trim this bit out, at least for an initial merge, 
but there may be other stuff it'd be helpful to hook in here.

> Either way, I think the hypervisor interface should support multiple
> entities per guest trying to set up a watchdog timer. Even if just so that
> entities that lose the race get -EBUSY. This would make it safer to use
> without a single controlling entity in the guest kernel.

Hmmm.  I could do something like that, certainly.  The model I was following 
was that of hardware - I'm not sure what a physical watchdog card would do 
with multiple controlling entities but I doubt it'd be pretty.  As we're in 
the hypervisor, we've got more smarts available to us though, so a bit of 
opaque state to identify a "watchdog handle" should be trivial to add.  I 
wonder if there are any potential users this sort of behaviour, though - 
maybe Richard would comment here?

The only significant feature I'd want to add before merge is support for 
suspend / resume - that should be fairly simple to add, though.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
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