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From: Paul Turner <pjt@google.com>
Date: Tue, 2 Oct 2012 14:14:14 -0700
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Subject: Re: [patch 11/16] sched: replace update_shares weight distribution
 with per-entity computation
To: Benjamin Segall <bsegall@google.com>
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        linux-kernel@vger.kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
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        Vincent Guittot <vincent.guittot@linaro.org>,
        Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>,
        Morten Rasmussen <Morten.Rasmussen@arm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Namhyung Kim <namhyung@kernel.org>
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On Mon, Sep 24, 2012 at 1:39 PM, Benjamin Segall <bsegall@google.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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