linux-kernel - Re: [patch 11/16] sched: replace update_shares weight distribution with per-entity computation

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Date:	Tue, 2 Oct 2012 14:14:14 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Benjamin Segall <bsegall@...gle.com>
Cc:	Jan H. Schönherr <schnhrr@...tu-berlin.de>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Venki Pallipadi <venki@...gle.com>,
	Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [patch 11/16] sched: replace update_shares weight distribution
 with per-entity computation

On Mon, Sep 24, 2012 at 1:39 PM, Benjamin Segall <bsegall@...gle.com> wrote:
> blocked_load_avg ~= \sum_child child.runnable_avg_sum/child.runnable_avg_period * child.weight
>
> The thought was: So if all the children have hit zero runnable_avg_sum
> (or in the case of a child task, will when they wake up), then
> blocked_avg sum should also hit zero at the same and we're in theory
> fine.
>
> However, child load can be significantly larger than even the maximum
> value of runnable_avg_sum (and you can get a full contribution off a new
> task with only one tick of runnable_avg_sum anyway...), so
> runnable_avg_sum can hit zero first due to rounding. We should case on
> runnable_avg_sum || blocked_load_avg.

Clipping blocked_load_avg when runnable_avg_sum goes to zero is
sufficient.  At this point we cannot contribute to our parent anyway.

>
>
> As a side note, currently decay_load uses SRR, which means none of these
> will hit zero anyway if updates occur more often than once per 32ms. I'm
> not sure how we missed /that/, but fixes incoming.

Egads, fixed.  We definitely used to have that, I think it got lost in
the "clean everything up, break it into a series, and make it pretty"
step.  Perhaps that explains why some of the numbers in the previous
table were a little different.


>
> Thanks,
> Ben
>
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