linux-kernel - Re: [RFC][-mm] Simple stats for cpu resource controller v3

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Message-Id: <20080502125304.77dd38ba.akpm@linux-foundation.org>
Date:	Fri, 2 May 2008 12:53:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Balaji Rao <balajirrao@...il.com>
Cc:	dhaval@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	containers@...ts.osdl.org, menage@...gle.com, balbir@...ibm.com,
	vatsa@...ux.vnet.ibm.com, a.p.zijlstra@...llo.nl
Subject: Re: [RFC][-mm] Simple stats for cpu resource controller v3

On Sat, 3 May 2008 01:10:28 +0530
Balaji Rao <balajirrao@...il.com> wrote:

> On Friday 02 May 2008 02:30:26 am Andrew Morton wrote:
> <snip>
> 
> Hi Andrew,
> 
> Thank you for the review.
> >
> > Did you consider using include/linux/percpu_counter.h?
> >
> > If so, what was wrong with it?
> >
> > Because it would be much better to fix per-cpu counters than to invent new
> > stuff.
> No, I hadn't consider using the percpu_counters infrastructure. But today when 
> I tried using it, I got an early exception.I guess its because I tried 
> calling percpu_counter_init from within sched_init, which I perhaps shouldn't 
> do, because percpu_counter_init expects cpu hotplug code to be initialized by 
> then. Right ? Correct me if I'm wrong.

I don't see any reason why we cannot run percpu_counter_init() prior to
running percpu_counter_startup().  And it is desirable that we be able to
start using the percpu-counters quite early.

Can you debug it a bit please?  It's probably some silly little thing,
perhaps fixable by calling percpu_counter_startup() earlier.

> How about we start collecting statistics at a later stage i.e, after 
> percpu_counter becomes usable ?

It would be better to make the core infrastructure more robust, rather
than working around problems it might have.


It's rather nice that percpu_counters internally take care of
cpu-hotplugging, and use cpu_online_map.  I was amazed at how easily that
was added.  I still expect it to break somehow..
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