Re: [PATCH] sched: Rationalize sys_sched_rr_get_interval()

[Peter Williams <pwil3058@bigpond.net.au>]

Jarek Poplawski wrote:
> On 13-10-2007 03:29, Peter Williams wrote:
> > Jarek Poplawski wrote:
> > > On 12-10-2007 00:23, Peter Williams wrote:
> > > ...
> > > > The reason I was going that route was for modularity (which helps 
> > > > when adding plugsched patches).  I'll submit a revised patch for 
> > > > consideration.
> > > ...
> > >
> > > IMHO, it looks like modularity could suck here:
> > >
> > > > +static unsigned int default_timeslice_fair(struct task_struct *p)
> > > > +{
> > > > +    return NS_TO_JIFFIES(sysctl_sched_min_granularity);
> > > > +}
> > > If it's needed for outside and sched_fair will use something else
> > > (to avoid double conversion) this could be misleading. Shouldn't
> > > this be kind of private and return something usable for the class
> > > mainly?
> > This is supplying data for a system call not something for internal use 
> > by the class.  As far as the sched_fair class is concerned this is just 
> > a (necessary - because it's need by a system call) diversion.
>
> So, now all is clear: this is the misleading case!
>
> > > Why anything else than sched_fair should care about this?
> > sched_fair doesn't care so if nothing else does why do we even have 
> > sys_sched_rr_get_interval()?  Is this whole function an anachronism that 
> > can be expunged?  I'm assuming that the reason it exists is that there 
> > are user space programs that use this system call.  Am I correct in this 
> > assumption?  Personally, I can't think of anything it would be useful 
> > for other than satisfying curiosity.
>
> Since this is for some special aim (not default for most classes, at
> least not for sched_fair) I'd suggest to change names:
> default_timeslice_fair() and .default_timeslice to something like eg.:
> rr_timeslice_fair() and .rr_timeslice or rr_interval_fair() and
> .rr_interval (maybe with this "default" before_"rr_" if necessary).
>
> On the other hand man (2) sched_rr_get_interval mentions that:
> "The identified process should be running under the SCHED_RR
> scheduling policy".
>
> Also this place seems to say about something simpler:
> http://www.gnu.org/software/libc/manual/html_node/Basic-Scheduling-Functions.html
>
> So, I still doubt sched_fair's "notion" of timeslices should be
> necessary here.

As do I.  Even more so now that you've shown me the man page for 
sched_rr_get_interval().

I'd suggest that we modify sched_rr_get_interval() to return -EINVAL 
(with *interval set to zero) if the target task is not SCHED_RR.  That 
way we can save a lot of unnecessary code.  I'll work on a patch. 
Unless you want to do it?

> Sorry for too harsh words.

I didn't consider them harsh.

Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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