Date:	Tue, 28 Oct 2008 20:41:34 +0100
From:	Henrik Austad <henrik@...tad.us>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	Dario <faggioli@...dalf.sssup.it>
Subject: Re: Rearranging layout of code in the scheduler

On Tuesday 28 October 2008 17:50:27 Peter Zijlstra wrote:
> On Tue, 2008-10-28 at 16:34 +0100, Henrik Austad wrote:
> > Hello,
> >
> > Before I dive in, I should probably justify my motivations for writing
> > this email. I'm working away on implementing an EDF scheduler for real
> > time tasks in the kernel. This again leads to hacking at the existing
> > source as I'm not about to toss out the entire scheduler - just replace
> > (by some Kconfig switch) the RR/FIFO classes. As to why I'm looking at
> > EDF, I think the answer to that is a bit too long (and not appropriate
> > for this email anyway) so I'll leave that part out.
>
> You and a few other folks. The most interesting part of EDF is not the
> actual scheduler itself (although there are fun issues with that as
> well), but extending the Priority Inheritance framework to deal with all
> the fun cases that come with EDF.

well, yes, you trade in priority inversion with deadline inversion. Probably a 
few other exotic issues as well :-)

> > However, what I do mean to discuss is the current state of the scheduler
> > code. Working through the code, I must say I'm really impressed. The
> > code is clean, it is well thought out and the new approach with
> > sched_class and sched_entity makes it very modular. However, digging
> > deeper, I find myself turning more and more desperate, looking over my
> > shoulder for a way out.
> >
> > Now, I'm in no doubt that the code *is* modular, that it *is* clean and
> > tidy, but coming from outside, it is not that easy to grasp it all. And,
> > it is not just the sheer size and complexity of the scheduler itself,
> > but also a lot with how the code is arranged.
> >
> > For instance, functions like free_fair_sched_group,
> > alloc_fair_sched_group etc - does IMHO belong in sched_fair.c and not in
> > sched.c. The same goes for several rt-functions and structs.
> >
> > So, if one drew up a list over all events that would cause functions in
> > sched.c to be called, this could be used to make a minimized 'interface'
> > and then let the scheduler call the appropriate function for the given
> > class in the same fashion sched_tick is used today.
>
> I'd start out small by moving the functions to the right file. After
> that you could look at providing methods in the sched_class.

Yes, I was thinking something along those lines, slowly moving things out. I 
just wanted to clear the air before I got started in case someone had some 
real issues with this approach. After all, if it's never going to get merged, 
there's no point doing it.

> > What I would like, is to rip out all the *actual* scheduling logic and
> > put this in sched_[fair|rt].c and let sched be purely event-driven
> > (which would be a nice design goal in itself). This would also lead to
> > sched_[fair|rt].h, where the structs, macros, defines etc can be
> > defined. Today these are defined in just about everywhere, making the
> > code unnecessary complicated (I'm not going to say messy since I'm not
> > *that* senior to kernel coding :-))
>
> You might need to be careful there, or introduce sched_(fair|rt).h for
> those.

This is a final goal, the light in the end of the tunnel if you like.

> > Why not use the sched_class for all that it's worth - make the different
> > classes implement a set of functions and let sched.c be oblivious to
> > what's going on other than turning the machinery around?
>
> Sounds good, its been on the agenda for a while, but nobody ever got
> around to it.
>
> Other cleanups that can be done are:
>  - get rid of all the load balance iterator stuff and move
>    that all into sched_fair

Ah, yes. I can give that a go

>  - extract the common sched_entity members and create:
>
>    struct {
> 	struct sched_entity_common common;
> 	union {
> 		struct sched_entity fair;
> 		struct sched_rt_entity rt;
> 	}
>    }
>
> > Is this something worth pursuing? I mean, the scheduler *does* work, and
> > if it ain't broken, don't fix it. However, I have a strong feeling that
> > this can be done a lot cleaner, not to mention, changing from one type
> > of scheduler to another will be much easier. :-)
>
> Well, adding a sched_class, no need to replace anything besides that.

I wasn't really aiming at replacing anything (besides the rt-stuff, but as 
said earlier, that's not the issuen ow). I just want to move things to make 
it a bit clearer. 

-- 
med Vennlig Hilsen - Yours Sincerely
Henrik Austad

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