linux-kernel - Re: [PATCH v5 5/7] sched: compute runnable load avg in cpu_load and cpu_avg_load_per

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Date:	Mon, 06 May 2013 23:00:47 +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>
Subject: Re: [PATCH v5 5/7] sched: compute runnable load avg in cpu_load and
 cpu_avg_load_per_task

> 
> blocked_load_avg is the expected "to wake" contribution from tasks
> already assigned to this rq.
> 
> e.g. this could be:
>   load = this_rq->cfs.runnable_load_avg + this_rq->cfs.blocked_load_avg;

Current load balance doesn't consider slept task's load which is
represented by blocked_load_avg. And the slept task is not on_rq, so
consider it in load balance is a little strange.

But your concern is worth to try. I will change the patchset and give
the testing results.

> 
> Although, in general I have a major concern with the current implementation:
> 
> The entire reason for stability with the bottom up averages is that
> when load migrates between cpus we are able to migrate it between the
> tracked sums.
> 
> Stuffing observed averages of these into the load_idxs loses that
> mobility; we will have to stall (as we do today for idx > 0) before we
> can recognize that a cpu's load has truly left it; this is a very
> similar problem to the need to stably track this for group shares
> computation.
> 
> To that end, I would rather see the load_idx disappear completely:
>  (a) We can calculate the imbalance purely from delta (runnable_avg +
> blocked_avg)
>  (b) It eliminates a bad tunable.

I also show the similar concern of load_idx months ago. seems overlooked. :)
> 
>> -       return cpu_rq(cpu)->load.weight;
>> +       return (unsigned long)cpu_rq(cpu)->cfs.runnable_load_avg;
> 
> Isn't this going to truncate on the 32-bit case?

I guess not, the old load.weight is unsigned long, and runnable_load_avg
is smaller than the load.weight. so it should be fine.

btw, according to above reason, guess move runnable_load_avg to
'unsigned long' type is ok, do you think so?
> 

-- 
Thanks
    Alex
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