Re: [PATCH v3 2/5] build: Hook the schedulers into Kconfig

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/01/16 17:04, Jonathan Creekmore wrote:
> Doug Goldstein writes:
>
>> On 1/8/16 10:49 AM, Jan Beulich wrote:
>>>>>> On 08.01.16 at 17:30, <jonathan.creekmore@gmail.com> wrote:
>>>> So, based on the Kconfig setup and the linker ASSERT, there should be no
>>>> way to have a default scheduler that is not in the build. I wish Kconfig
>>>> allowed you to state that you must have at least one option selected or
>>>> up to all of the options selected (so I could require 1-4 schedulers
>>>> compiled in, but not 0). Unfortunately, the only way that seems allowed
>>>> is if you compile code in as modules, which Xen does not (with a choice
>>>> block, you can select multiple items as M, but only 1 as Y, but at least
>>>> 1 must be present --- since we only support Y, the choice block wouldn't
>>>> work for the schedulers).
>>> Perhaps credit should just not be configurable then?
>>>
>>> Jan
>>>
>> How much effort are we willing to put in saving someone? We've already
>> got these options completely hidden away. We've already given the user a
>> warning that what they're doing isn't supported and they shouldn't be
>> touching this. Then they have to disable the scheduler that they pick as
>> the default. How many people are we expecting to really do this? I'm all
>> for idiot proofing things but at some point we've got to say there's
>> enough barriers to prevent this from happening. Otherwise we need to put
>> a Xen developer in every Xen users home or office.
> At this point, with the (not submitted yet) ASSERT in the linker file,
> it is a compile error to not have any schedulers linked in and Kconfig
> enforces that only schedulers compiled in can be a default.

I think that should be sufficient.  A user has to go a long way out of
their way to get into this situation in the first place.


If there is a nice way for checking for an empty string in the
preprocessor, then something like

BUILD_BUG_ON(ARRAY_SIZE(CONFIG_SCHED_DEFAULT) == 1);

would also be a good sanity check, but I doubt this specific example
compiles.

~Andrew