Re: [PATCH 4/8] sched: clean up move_task() and move_one_task()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2013-03-20 at 16:33 +0900, Joonsoo Kim wrote:

> > Right, so I'm not so taken with this one. The whole load stuff really
> > is a balance heuristic that's part of move_tasks(), move_one_task()
> > really doesn't care about that.
> > 
> > So why did you include it? Purely so you didn't have to re-order the
> > tests? I don't see any reason not to flip a tests around.
> 
> I think that I'm not fully understand what you are concerning, because of
> my poor English. If possible, please elaborate on a problem in more detail.

OK, so your initial Changelog said it wanted to remove some code
duplication between move_tasks() and move_one_task(); but then you put
in the load heuristics and add a boolean argument to only enable those
for move_tasks() -- so clearly that wasn't duplicated.

So why move that code.. I proposed that this was due a reluctance to
re-arrange the various tests that stop the migration from happening.

Now you say:

> ... Just moving up can_migrate_task() above
> load evaluation code may raise side effect, because can_migrate_task() have
> other checking which is 'cache hottness'. I don't want a side effect. So
> embedding load evaluation to can_migrate_task() and re-order checking and
> makes load evaluation disabled for move_one_task().

Which pretty much affirms this. However I also said that I don't think
the order really matters that much; each test will cancel the migration
of this task; the order of these tests seem immaterial.

> If your recommandation is to move up can_mirate_task() above
> load evaluation code, yes, I can, and will do that. :)

I would actually propose