Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost whenever newidle balance is attempted

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 24, 2014 at 09:53:37AM -0700, Jason Low wrote:
> 
> So I thought that the original rationale (commit 1bd77f2d) behind
> updating rq->next_balance in idle_balance() is that, if we are going
> idle (!pulled_task), we want to ensure that the next_balance gets
> calculated without the busy_factor.
> 
> If the rq is busy, then rq->next_balance gets updated based on
> sd->interval * busy_factor. However, when the rq goes from "busy"
> to idle, rq->next_balance might still have been calculated under
> the assumption that the rq is busy. Thus, if we are going idle, we
> would then properly update next_balance without the busy factor
> if we update when !pulled_task.
> 

Its late here and I'm confused!

So the for_each_domain() loop calculates a new next_balance based on
->balance_interval (which has that busy_factor on, right).

But if it fails to pull anything, we'll (potentially) iterate the entire
tree up to the largest domain; and supposedly set next_balanced to the
largest possible interval.

So when we go from busy to idle (!pulled_task), we actually set
->next_balance to the longest interval. Whereas the commit you
referenced says it sets it to a shorter while.

Not seeing it.

So the code as modified by Ingo in one of the initial CFS commits, will
move the ->next_balance time ahead if the balance succeeded
(pulled_task), thereby reflecting that we are busy and we just did a
balance so we need not do one again soon. (we might want to re-think
this if we really make the idle balance only pull 1 task max).

Of course, I've now gone over this code 3 times today, so I'm terminally
confused.