linux-kernel - Re: [RFC] cgroup TODOs

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Message-ID: <20120914194439.GP17747@google.com>
Date:	Fri, 14 Sep 2012 12:44:39 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
	Michal Hocko <mhocko@...e.cz>,
	Glauber Costa <glommer@...allels.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Graf <tgraf@...g.ch>, Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Neil Horman <nhorman@...driver.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>
Subject: Re: [RFC] cgroup TODOs

Hello, Vivek.

On Fri, Sep 14, 2012 at 03:28:40PM -0400, Vivek Goyal wrote:
> Hmm.., In that case how libvirt will make use of blkio in the proposed
> scheme. We can't disable blkio nesting at "system" level. So We will
> have to disable it at each service level  except "libvirtd" so that
> libvirt can use blkio for its virtual machines.
> 
> That means blkio will see each service in a cgroup of its own and if 
> that does not make sense by default, its a problem. In the existing

Yeap, if libvirtd wants use blkcg, blkcg will be enabled upto
libvirtd's root.  It might not be optimal but I think it makes sense.
If you want to excercise hierarchical control on a resource, the only
sane way is sticking to the hierarchy until it reaches root.

> scheme, atleast every service does not show up in its cgroup from
> blkio point of view. Everthig is in root and libvirt can create its
> own cgroups, keeping number of cgroups small.

Even a broken clock is right twice a day.  I don't think this is a
behavior we can keep for the sake of "but if we do this ass-weird
thing, we can bypass the overhead for XYZ" when it breaks so many
fundamental things.

I think there currently is too much (broken) flexibility and intent to
remove it.  That doesn't mean that removeing all flexibility is the
right direction.  It inherently is a balancing act and I think the
proposed solution is a reasonable tradeoff.  There's important
difference between causing full overhead by default for all users and
requiring some overhead when the use case at hand calls for the
functionality.

Thanks.

-- 
tejun
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