Re: [Xen-devel] [PATCH 3/3] tools: sched: add support for 'null' scheduler

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 06/04/17 11:49, Dario Faggioli wrote:
> On Mon, 2017-03-27 at 11:50 +0100, George Dunlap wrote:
>> On 17/03/17 18:43, Dario Faggioli wrote:
>>> --- a/tools/libxl/libxl_sched.c
>>> +++ b/tools/libxl/libxl_sched.c
>>> @@ -178,6 +178,20 @@ static int sched_arinc653_domain_set(libxl__gc
>>> *gc, uint32_t domid,
>>>      return 0;
>>>  }
>>>  
>>> +static int sched_null_domain_set(libxl__gc *gc, uint32_t domid,
>>> +                                 const libxl_domain_sched_params
>>> *scinfo)
>>> +{
>>> +    /* The null scheduler doesn't take any domain-specific
>>> parameters. */
>>> +    return 0;
>>> +}
>>> +
>>> +static int sched_null_domain_get(libxl__gc *gc, uint32_t domid,
>>> +                               libxl_domain_sched_params *scinfo)
>>> +{
>>> +    /* The null scheduler doesn't have any domain-specific
>>> parameters. */
>>> +    return ERROR_INVAL;
>>> +}
>>
>> Why the different return value?  Why not return either INVAL or
>> SUCCESS
>> for both?
>>
> Because domain_set() is called by libxl_domain_sched_params_set(),
> which is in turn called unconditionally within libxl__build_post(),
> with the purpose of setting the scheduling parameters chosen by the
> user during domain creation.
> 
> If that fails (I've tried that), domain creation fails too. So either
> it returns success, or we'd have to modify (at least)
> liblx__build_post(), teaching it about acceptable failures.
> 
> OTOH, we indeed could return success for domain_get() too, for the sake
> of having the two above functions return the same. But I really think
> that call should fail, as an indication to the callers that they won't
> get the value of any parameter for this scheduler.

I see.  So if *our* code doesn't know that there aren't any parameters
to set, that's OK; but if *other people's code doesn't know that there
aren't any parameters to get, it needs to be changed to know that.  Got
it. ;-)

There is a sort of mathematical logic to the idea that setting a null
set of parameters should always succeed; and it's certainly convenient
for tools to be able to always just call libxl_domain_sched_params_set()
without having to check what scheduler is there.  But the same logic I
think applies to get(), so I would say to return 0 for both.

But Wei and Ian have the final say.

 -George


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