On Wed, Nov 27, 2013 at 03:01:43PM +0100, Ingo Molnar wrote: > > 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. > > There are two approaches to this that I can see: > > 1) > > allow partial information to be returned to user-space, for existing > input parameters. The new fields won't be displayed, but the tool > doesn't know about them anyway so it's OK. The tool can still display > all the other existing parameters.
But suppose a task simply wants to lower/raise its static (FIFO) priority and does: sched_getparam(¶ms); params.prio += 1; sched_setparam(¶ms); If anything outside of the known param fields was set, we just silently lost it, for the setparam() call will fill out 0s for the unprovided fields. > 2) > > Return -ENOSYS if the 'extra' fields are nonzero. In this case the > usual case of old tooling + new kernel will still work just fine, > because old tooling won't set the new fields to any non-default > (nonzero) values. In the 'mixed' case old tooling will not be able to > change/display those fields. > > I tend to lean towards #1. What do you think? As per the above that can result in silent unexpected behavioural changes. I'd much rather be explicit and break hard; so 2). So mixing new tools (schedtool, chrt etc) and old apps will give pain, but at least not silent surprises. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
