Re: SCHED_DEADLINE, sched_getscheduler(), and sched_getparam()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 12, 2014 at 02:09:58PM +0200, Michael Kerrisk (man-pages) wrote:
> Hi Peter,
> 
> Looking at the code of sched_getparam() and sched_setscheduler() (to
> see what might need to land in the man pagea with respect to
> SCHED_DEADLINE changes), I see that the former fails (EINVAL) if the
> target is a SCHED_DEADLINE process, while the latter succeeds
> (returning SCHED_DEADLINE).
> 
> The sched_setscheduler() seems fine, but what's the rationale for
> having sched_getparam() fail in this case, rather than just returning
> a sched_priority of zero (since sched_priority is in any case unused,
> as for SCHED_OTHER, right)? My point is that the change seems to
> needlessly break applications that employ sched_getparam(). Maybe I am
> missing something...

s/setscheduler/getscheduler/ ?

I'm a proponent of fail hard instead of fail silently and muddle on.

And while we can fully and correctly return sched_getscheduler() we
cannot do so for sched_getparam().

Returning sched_param::sched_priority == 0 for DEADLINE would also break
the symmetry between sched_setparam() and sched_getparam(), both will
fail for SCHED_DEADLINE.