Re: PV-vNUMA issue: topology is misinterpreted by the guest

[Juergen Gross <jgross@suse.com>]

On 07/22/2015 03:58 PM, Boris Ostrovsky wrote:
> On 07/22/2015 09:50 AM, Juergen Gross wrote:
>> On 07/22/2015 03:36 PM, Dario Faggioli wrote:
>>> On Tue, 2015-07-21 at 16:00 -0400, Boris Ostrovsky wrote:
>>>> On 07/20/2015 10:43 AM, Boris Ostrovsky wrote:
>>>>> On 07/20/2015 10:09 AM, Dario Faggioli wrote:
>>>
>>>>> I'll need to see how LLC IDs are calculated, probably also from some
>>>>> CPUID bits.
>>>>
>>>>
>>>> No, can't do this: LLC is calculated from CPUID leaf 4 (on Intel) which
>>>> use indexes in ECX register and xl syntax doesn't allow you to override
>>>> CPUIDs for such leaves.
>>>>
>>> Right. Which leaves us with the question of what should we do and/or
>>> recommend users to do?
>>>
>>> If there were a workaround that we could put in place, and document
>>> somewhere, however tricky it was, I'd say to go for it, and call it
>>> acceptable for now.
>>>
>>> But, if there isn't, should we disable PV vnuma, or warn the user that
>>> he may see issues? Can we identify, in Xen or in toolstack, whether an
>>> host topology will be problematic, and disable/warn in those cases too?
>>>
>>> I'm not sure, honestly. Disabling looks too aggressive, but it's an
>>> issue I wouldn't like an user to be facing, without at least being
>>> informed of the possibility... so, perhaps a (set of) warning(s)?
>>> Thoughts?
>>
>> I think we have 2 possible solutions:
>>
>> 1. Try to handle this all in the hypervisor via CPUID mangling.
>>
>> 2. Add PV-topology support to the guest and indicate this capability via
>>    elfnote; only enable PV-numa if this note is present.
>>
>> I'd prefer the second solution. If you are okay with this, I'd try to do
>> some patches for the pvops kernel.
>
> What if I configure a guest to follow HW topology? I.e. I pin VCPUs to
> appropriate cores/threads? With elfnote I am stuck with disabled topology.

Add an option to do exactly that: follow HW topology (pin vcpus,
configure vnuma)?

Add a force flag to the vnuma configuration to ignore the elfnote?

> Besides, this is not necessarily a NUMA-only issue, it's a scheduling
> one (inside the guest) as well.

Sure. That's what Jan said regarding SUSE's xen-kernel. No toplogy info
(or a trivial one) might be better than the wrong one...

This patch for pvops should be written in any case. I'll do this, but it
would be nice to know whether PV-numa should be considered or not.


Juergen