linux-kernel - Re: RFC: cgroups aware proc

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Message-ID: <52D48BA6.2080701@yuhu.biz>
Date:	Tue, 14 Jan 2014 02:58:14 +0200
From:	Marian Marinov <mm@...u.biz>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	lxc-devel@...ts.linuxcontainers.org, cgroups@...r.kernel.org,
	"Daniel P. Berrange" <berrange@...hat.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Li Zefan <lizefan@...wei.com>, Ingo Molnar <mingo@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: RFC: cgroups aware proc

On 01/13/2014 07:12 PM, Peter Zijlstra wrote:
> On Mon, Jan 13, 2014 at 06:23:50PM +0200, Marian Marinov wrote:
>> Hello Peter,
>>
>> I need help with the scheduler.
>>
>> I'm currently trying to patch the /proc/loadavg to show the load that is
>> only related to the processes from the current cgroup.
>>
>> I looked trough the code and I was hoping that tsk->sched_task_group->cfs_rq
>> struct will give me the needed information, but unfortunately for me, it did
>> not.
>>
>> Can you advise me, how to approach this problem?
>
> Yeah, don't :-) Really, loadavg is a stupid metric.

Yes... stupid, but unfortunately everyone is looking at it :(

>
>> I'm totally new to the scheduler code.
>
> Luckily you won't actually have to touch much of it. Most of the actual
> loadavg code lives in the first ~400 lines of kernel/sched/proc.c, read
> and weep. Its one of the best documented bits around.

I looked trough it but I don't understand how to introduce the per cgroup calculation.

I looked trough the headers and found the following, which is already implemented.

   task->sched_task_group->load_avg
   task->sched_task_group->cfs_rq->load_avg
   task->sched_task_group->cfs_rq->load.weight
   task->sched_task_group->cfs_rq->runnable_load_avg

Unfortunately there is almost no documentation for these elements of the cfs_rq and task_group structs.

It seams to me that part of the per task group loadavg code is already present.

>
> Your proposition however is extremely expensive, you turn something
> that's already expensive O(nr_cpus) into something O(nr_cpus *
> nr_cgroups).
>
> I'm fairly sure people will not like that, esp. for something of such
> questionable use as the loadavg -- its really only a pretty number that
> doesn't mean all that much.

I know that its use is questionable but in my case I need to have it, or I will not be able to offer correct loadavg 
values in the containers.


>
>> -------- Original Message --------
>> From: Li Zefan <lizefan@...wei.com>
>>
>> Then you should add Peter, Ingo and LKML to your Cc list. :)
>
> You failed that, let me fix that.
>
>

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