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Date: Fri, 14 Sep 2012 12:44:39 -0700
From: Tejun Heo <tj@kernel.org>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: containers@lists.linux-foundation.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, Li Zefan <lizefan@huawei.com>,
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        Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Neil Horman <nhorman@tuxdriver.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Serge Hallyn <serge.hallyn@ubuntu.com>
Subject: Re: [RFC] cgroup TODOs
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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