On 05/12/2014 05:25 PM, Peter Zijlstra wrote: > On Mon, May 12, 2014 at 02:33:42PM +0200, Michael Kerrisk (man-pages) wrote: >>> 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. >> >> Maybe. But there seems to me to be a problem with your logic here. >> (And the symmetry argument seems a weak one to me.) >> >> I mean, applications that are currently using sched_getscheduler() >> will now get back a new policy (SCHED_DEADLINE) that they may not >> understand, and so they may break. >> >> On the other hand, applications that call sched_getparam() will fail >> with EINVAL, even though sched_priority has no meaning for >> SCHED_DEADLINE (as for the non-real-time policies), and so it would >> seem to be harmless to succeed and return a sched_priority of 0 in >> this case. It seems to break user-space needlessly, IMHO. >> >> If anything, I'd have said it would have made more sense to have the >> sched_getscheduler() case fail, while having the sched_getparam() case >> succeed. (But, I can see the argument for having _both_ cases >> succeed.) > > Hmm,.. maybe. Can we still change this? Again, maybe, there's not really > that much userspace that relies on this. I think the sched_getparam() change is worthwhile (and the patches could (should?) be marked for -stable). I suspect there's no user space that relies on the current SCHED_DEADLINE behavior, and it's worth avoiding the above breakage for sched_getparam(). I'd be inclined to leave sched_getscheduler() as is: there's arguments either way for how it should behave. > In any case, the way I read the little there is on getparam() it seems > to imply the only case where it does make sense to call it at all is > when sched_getscheduler() returns either SCHED_FIFO or SCHED_RR. (Yes, that's my understanding too.) > And in that sense I suppose the precedent for all other currently > available classes to not fail the param call but return 0 should be > extended. Yes. > If only we'd started out with sched_yield()/sched_getparam() etc failing > when not !SCHED_FIFO/RR :-) Here, I think we're just following POSIX. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
