Re: [PATCH 02/14] sched: add extended scheduling interface. (new ABI)

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 27, 2013 at 02:23:54PM +0100, Ingo Molnar wrote:
> There are 3 main compatibility cases:
> 
>  - the kernel's 'sizeof sched_attr' is equal to sched_attr:size: the 
>    kernel version and user-space version matches, it's a straight ABI 
>    in this case with full functionality.
> 
>  - the kernel's 'sizeof sched_attr' is larger than sched_attr::size 
>    [the kernel is newer than what user-space was built for], in this 
>    case the kernel assumes that all remaining values are zero and acts
>    accordingly.

It also needs to fail sched_getparam() when any of the fields that do
not fit in the smaller struct provided are !0.

>  - the kernel's 'sizeof sched_attr' is smaller than sched_attr::size 
>    [the kernel is older than what user-space was built for]. In this 
>    case the kernel should return -ENOSYS if any of the additional 
>    fields are nonzero. If those are all zero then it will work as if a 
>    smaller structure was passed in.

So the problem I see with this one is that because you're allowed to
call sched_setparam() or whatever it will be called next on another
task; a task can very easily fail its sched_getparam() call.

Suppose the application is 'old' and only supports a subset of the
fields; but its wants to get, modify and set its params. This will work
as long nothing will set anything it doesn't know about.

As soon as some external entity -- say a sysad using schedtool -- sets a
param field it doesn't support the get, modify, set routing completely
fails.