linux-kernel - Re: [RESEND PATCH V5 0/8] remove cpu

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Date:	Thu, 08 May 2014 14:40:59 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Preeti Murthy <preeti.lkml@...il.com>,
	Alex Shi <alex.shi@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>, mingo@...hat.com,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Mike Galbraith <efault@....de>, wangyun@...ux.vnet.ibm.com,
	LKML <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: [RESEND PATCH V5 0/8] remove cpu_load idx

On 05/06/2014 05:09 PM, Peter Zijlstra wrote:
> On Tue, May 06, 2014 at 03:24:13PM +0530, Preeti Murthy wrote:
>> Hi Morten, Peter, Alex,
>>
>> In a similar context, I noticed that /proc/loadavg makes use of
>> avenrun[] array which keeps track of the history of the global
>> load average. This however makes use of the sum of
>> nr_running + nr_uninterruptible per cpu. Why are we not
>> using the cpu_load[] array here which also keeps track
>> of the history of per-cpu load and then return a sum of it?
> 
> Entirely different kind of 'load'. Note that you cannot use
> ->nr_uninterruptible per-cpu, also note that sched/proc.c doesn't.

True, I worded it wrong above. It uses the per_cpu nr_running and a
global nr_uninterruptible.

> 
>> Using nr_running to show the global load average would
>> be misleading when entire load balancing is being done on the
>> basis of the history of cfs_rq->runnable_load_avg/cpu_load[]
>> right? IOW, to the best of my understanding we do not use
>> nr_running anywhere to directly determine cpu load in the kernel.
>>
>> My idea was that the global/per_cpu load that we reflect via
>> proc/sys interfaces must be consistent. I haven't really
>> looked at what /proc/schedstat, /proc/stat, top are all reading
>> from. But /proc/loadavg is reading out global nr_running +
>> waiting tasks when this will not give us the accurate picture
>> of the system load especially when there are many short running
>> tasks.
> 
> Nobody said /proc/loadavg is a sane number, but its what it is and since
> its a global number its entirely unsuited for balancing -- not to
> mention all other reasons its crap.

I agree its not meant for balancing. My point was that since its
inaccurate why don't we correct it. But if your argument is that we can
live with /proc/loadavg showing a reasonable view of system load then it
shouldn't be a problem.

Regards
Preeti U Murthy
> 

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