Messages in this thread | | | Date | Wed, 27 Nov 2013 15:17:21 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 02/14] sched: add extended scheduling interface. (new ABI) |
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* Peter Zijlstra <peterz@infradead.org> wrote:
> 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.
You are right, I concur.
Thanks,
Ingo
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