Re: [PATCH v2 2/3] xen: add hypercall option to temporarily pin a vcpu

[Juergen Gross <jgross@suse.com>]

On 02/03/16 17:03, Dario Faggioli wrote:
> On Wed, 2016-03-02 at 16:34 +0100, Juergen Gross wrote:
>> On 02/03/16 10:27, Dario Faggioli wrote:
>>>  
>>> However, an xl flag is easier to add, easier to document and easier
>>> and
>>> more natural to find, from the point of view of an user that really
>>> needs it. And perhaps it could turn out useful for other situations
>>> in
>>> future. So, I guess I'd say:
>>>  - yes, let's add that
>>>  - let's do it as a "force flag" of `xl vcpu-pin'.
>> Which raises the question: how to do that on the libxl level?
>>
> Ah, right.
> 
>> a) expand libxl_set_vcpuaffinity() with another parameter (is this
>> even
>>    possible? I could do some ifdeffery, but the API would change...)
>>
>> b) add a libxl_set_vcpuaffinity_force() variant
>>
>> c) imply the force flag by specifying both hard and soft maps as NULL
>>    (it _is_ basically just that: keep both affinity sets), implying
>> that
>>    it makes no sense to specify any affinities with the -f flag
>> (which
>>    renders the "force" meaning rather strange, would be more a
>> "restore"
>>    now).
>>
> Eheh, tools' maintainers' call. My preference would be b).
> 
> I don't like a), mostly because that would mean everyone will need to
> specify a parameter that it is really only necessary in special cases.
> 
> I could live with c), but it indeed makes the semantic too convoluted
> for my taste.
> 
> I guess, however, that even if going for b), we need to decide whether
> to require a cpumask or not, and what to do if one passes NULL. Maybe
> we can have a cpumask parameter and,
>  - if it is not NULL, force affinity to that,
>  - if it is NULL, just 'restore';
> what do you think?

I would just let the force flag restore the old setting (thus clearing
the affinity_broken flag) and then apply the normal affinity settings.

> Actually, at Xen level, the override only acts on hard affinity...
> should libxl take only one cpumask (for hard affinity only), or both
> hard and soft?

Just as the user is specifying: 0, 1 or 2.

> I'd say just one for hard is enough, unless we want to make space for a
> potential future situation where we will want to break and restore soft
> affinity as well...

The force flag would be just an add-on. That's rather easy in the
hypervisor and in the tools.


Juergen


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