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

[Ingo Molnar <mingo@kernel.org>]

* 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(&params);
> params.prio += 1;
> sched_setparam(&params);
> 
> 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