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

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Message-ID: <4E7794B1.5030907@parallels.com>
Date:	Mon, 19 Sep 2011 16:14:57 -0300
From:	Glauber Costa <glommer@...allels.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	<linux-kernel@...r.kernel.org>, <xemul@...allels.com>,
	<paul@...lmenage.org>, <lizf@...fujitsu.com>,
	<daniel.lezcano@...e.fr>, <jbottomley@...allels.com>
Subject: Re: [PATCH 1/9] Remove parent field in cpuacct cgroup

On 09/19/2011 03:44 PM, Peter Zijlstra wrote:
> 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 ;-)

Well, sure. But then, the only thing I am doing in this particular hunk 
is changing the definition of how to grab a parent... Can easily even be 
left aside.

>>> 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 ;-)

Well, thinking again about that, maybe it is not that much of a good 
idea. systemd is already starting to span a lot of cgroups... and if we 
update the tree bottom-up, we actually do have a much larger cost, since 
we need to touch all its breath...
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