linux-kernel - Re: [PATCH 1/9] Remove parent field in cpuacct cgroup

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Date:	Mon, 19 Sep 2011 20:44:28 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, xemul@...allels.com,
	paul@...lmenage.org, lizf@...fujitsu.com, daniel.lezcano@...e.fr,
	mingo@...e.hu, jbottomley@...allels.com
Subject: Re: [PATCH 1/9] Remove parent field in cpuacct cgroup

On Mon, 2011-09-19 at 15:38 -0300, Glauber Costa wrote:
> On 09/19/2011 03:35 PM, Peter Zijlstra wrote:
> > On Mon, 2011-09-19 at 13:30 -0300, Glauber Costa wrote:
> >> For cpuusage, I am not sure this optimization is a valid one
> >
> > I was talking about cpuusage, cpuacct_charge() is called for every
> > ctxsw/tick.
> 
> I am not touching it right now.

See hunk #4 of this particular patch:

@@ -9302,7 +9308,7 @@ static void cpuacct_charge(struct task_struct *tsk, u64 cputime)

That's cpuusage muck ;-)

> > But even for cpuacct tick stuff, wouldn't you need to sum all your child
> > cgroups to update the current cgroup? and that up the whole tree?
> 
> Of course I would. But as I said, it does not need to be done every 
> tick, in case it poses such a cacheline mayhem as you fear.
> 
> Since we'll only really need those values when someone reads it - which 
> is a far less frequent operation than the tick resolution - and when a 
> cgroup is destroyed - even less frequent operation - it should work well.

Quite possible, yes. Although if you create a cgroup with 100 subgroups
and poll very frequently.. it all depends on the avg use case etc.. and
since I don't use any of this stuff someone needs to tell me about how
the trade-offs work out in practice.

So explicit changelogs with numbers and agreements from multiple users
go a long way to make me feel good ;-)
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