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

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Date:	Sat, 3 May 2008 04:17:03 +0530
From:	Balaji Rao <balajirrao@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 Saturday 03 May 2008 01:23:04 am Andrew Morton wrote:
> 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.
>
percpu_counter_init uses kmalloc to create percpu counters. This raises an 
early exception as kmem_cache is not initialized that early.

It worked for me if we statically allocate memory for the counters. But its 
not at all a nice thing to do and I don't see another way to make it fit for 
early use.

I'm beginning to run out of ideas! Why not do what I earlier suggested - begin 
collecting statistics once we are able to safely use percpu_counters ? This 
now seems to be the best alternative IMHO.

-- 
Warm Regards,

Balaji Rao
Dept. of Mechanical Engineering
NITK
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