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

[Peter Zijlstra]

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.
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