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

[Jason Low <jason.low2@hp.com>]

On Thu, 2014-04-24 at 14:44 +0200, Peter Zijlstra wrote:
> On Thu, Apr 24, 2014 at 02:04:15PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 24, 2014 at 03:44:47PM +0530, Preeti U Murthy wrote:
> > > What about the update of next_balance field? See the code snippet below.
> > > This will also be skipped as a consequence of the commit e5fc6611 right?
> > > 
> > > 	   if (pulled_task || time_after(jiffies, this_rq->next_balance)) {
> > >                  /*
> > >                   * We are going idle. next_balance may be set based on
> > >                   * a busy processor. So reset next_balance.
> > >                   */
> > >                  this_rq->next_balance = next_balance;
> > >          }
> > > 
> > > Also the comment in the above snippet does not look right to me.
> > > It says "we are going idle" but the condition checks for pulled_task.
> > 
> > Yeah, that's odd indeed. Ingo did that back in dd41f596cda0d, I suspect
> > its an error, but..
> > 
> > So I think that should become !pulled_task || time_after().
> 
> Hmm, no, I missed that the for_each_domain() loop pushes next_balance
> ahead if it did a balance on the domain.
> 
> So it actually makes sense and the comment is wrong, but then you're
> also right that we want to not skip that.

Hi Preeti, Peter,

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.