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

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Message-ID: <xm26vcf3b1fb.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Mon, 24 Sep 2012 13:39:36 -0700
From:	Benjamin Segall <bsegall@...gle.com>
To:	Jan H. Schönherr <schnhrr@...tu-berlin.de>
Cc:	pjt@...gle.com, 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

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.

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.

Thanks,
Ben

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