linux-kernel - Re: [patch v8 9/9] sched/tg: remove blocked_load

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Date:	Thu, 20 Jun 2013 09:33:18 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Paul Turner <pjt@...gle.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Changlong Xie <changlongx.xie@...el.com>, sgruszka@...hat.com,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [patch v8 9/9] sched/tg: remove blocked_load_avg in balance

On 06/17/2013 08:20 PM, Paul Turner wrote:
> On Fri, Jun 7, 2013 at 12:20 AM, Alex Shi <alex.shi@...el.com> wrote:
>> > blocked_load_avg sometime is too heavy and far bigger than runnable load
>> > avg, that make balance make wrong decision. So remove it.
> Ok so this is  going to have terrible effects on the correctness of
> shares distribution; I'm fairly opposed to it in its present form.
> 
> So let's see, what could be happening..
> 
> In  "sched: compute runnable load avg in cpu_load and
> cpu_avg_load_per_task" you already update the load average weights
> solely based on current runnable load.  While this is generally poor
> for stability (and I suspect the benefit is coming largely from
> weighted_cpuload() where you do want to use runnable_load_avg and not
> get_rq_runnable_load() where I suspect including blocked_load_avg() is
> correct in the longer term).

If the 'poor stability' means your previous example of 2 40% busy task
and one 90% busy task. It occasionally happens. but at least in all
testing, kbuild, aim7, tbench, oltp, hackbench, ltp etc. involve
blocked_load_avg is just worse, guess due to above reason.
> 
> Ah so.. I have an inkling:
>   Inside weighted_cpuload() where you're trying to use only
> runnable_load_avg; this is in-fact still including blocked_load_avg
> for a cgroup since in the cgroup case a group entities' contribution
> is a function of both runnable and blocked load.

with this patch tg will not include blocked_load_avg.

Honestly, blocked_load_avg should has its meaning, like in your
scenario. but just now, we only can see it bring more harm without any
help on all we tested benchmarks.
I can't find a reason to enable sth that hurt performance.
> 
> Having weighted_cpuload() pull rq->load (possibly moderated by
> rq->avg) would reasonably avoid this since issued shares are
> calculated using instantaneous weights, without breaking the actual
> model for how much load overall that we believe the group has.
> 

I considered to use rq->avg in weighted_cpuload, but when we do
move_tasks to balance load between cpu, we just consider the cfs tasks
not rt task, consider rq->load/avg will involved a unnecessary rt
interference. So I changed to cfs load only.

-- 
Thanks
    Alex
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