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SubjectRe: [RFC] blk-cgroup: Allow creation of hierarchical cgroups
On Tue, Nov 02, 2010 at 06:20:30PM -0400, Vivek Goyal wrote:
> o Allow hierarchical cgroup creation for blkio controller
>
> o Currently we disallow it as both the io controller policies (throttling
> as well as proportion bandwidth) do not support hierarhical accounting
> and control. But the flip side is that blkio controller can not be used with
> libvirt as libvirt creates a cgroup hierarchy deeper than 1 level.
>
> <top-level-cgroup-dir>/<controller>/libvirt/qemu/<virtual-machine-groups>
>
> o So this patch will allow creation of cgroup hierarhcy but at the backend
> everything will be treated as flat. So if somebody created a an hierarchy
> like as follows.
>
> root
> / \
> test1 test2
> |
> test3
>
> CFQ and throttling will practically treat all groups at same level.
>
> pivot
> / | \ \
> root test1 test2 test3
>
> o Once we have actual support for hierarchical accounting and control
> then we can introduce another cgroup tunable file "blkio.use_hierarchy"
> which will be 0 by default but if user wants to enforce hierarhical
> control then it can be set to 1. This way there should not be any
> ABI problems down the line.
>
> o The only not so pretty part is introduction of extra file "use_hierarchy"
> down the line. Kame-san had mentioned that hierarhical accounting is
> expensive in memory controller hence they keep it off by default. I
> suspect same will be the case for IO controller also as for each IO
> completion we shall have to account IO through hierarchy up to the root.
> if yes, then it probably is not a very bad idea to introduce this extra
> file so that it will be used only when somebody needs it and some people
> might enable hierarchy only in part of the hierarchy.
>
> o This is how basically memory controller also uses "use_hierarhcy" and
> they also allowed creation of hierarchies when actual backend support
> was not available.
>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---

Hi Jens,

Do you have any concerns about this patch? If not, can you please apply
it.

Thanks
Vivek

> Documentation/cgroups/blkio-controller.txt | 27 +++++++++++++++++++++++++++
> block/blk-cgroup.c | 4 ----
> 2 files changed, 27 insertions(+), 4 deletions(-)
>
> Index: linux-2.6/block/blk-cgroup.c
> ===================================================================
> --- linux-2.6.orig/block/blk-cgroup.c 2010-10-28 14:19:02.000000000 -0400
> +++ linux-2.6/block/blk-cgroup.c 2010-11-02 13:10:13.000000000 -0400
> @@ -1452,10 +1452,6 @@ blkiocg_create(struct cgroup_subsys *sub
> goto done;
> }
>
> - /* Currently we do not support hierarchy deeper than two level (0,1) */
> - if (parent != cgroup->top_cgroup)
> - return ERR_PTR(-EPERM);
> -
> blkcg = kzalloc(sizeof(*blkcg), GFP_KERNEL);
> if (!blkcg)
> return ERR_PTR(-ENOMEM);
> Index: linux-2.6/Documentation/cgroups/blkio-controller.txt
> ===================================================================
> --- linux-2.6.orig/Documentation/cgroups/blkio-controller.txt 2010-10-28 14:19:01.000000000 -0400
> +++ linux-2.6/Documentation/cgroups/blkio-controller.txt 2010-11-02 17:51:52.000000000 -0400
> @@ -89,6 +89,33 @@ Throttling/Upper Limit policy
>
> Limits for writes can be put using blkio.write_bps_device file.
>
> +Hierarchical Cgroups
> +====================
> +- Currently none of the IO control policy supports hierarhical groups. But
> + cgroup interface does allow creation of hierarhical cgroups and internally
> + IO policies treat them as flat hierarchy.
> +
> + So this patch will allow creation of cgroup hierarhcy but at the backend
> + everything will be treated as flat. So if somebody created a hierarchy like
> + as follows.
> +
> + root
> + / \
> + test1 test2
> + |
> + test3
> +
> + CFQ and throttling will practically treat all groups at same level.
> +
> + pivot
> + / | \ \
> + root test1 test2 test3
> +
> + Down the line we can implement hierarchical accounting/control support
> + and also introduce a new cgroup file "use_hierarchy" which will control
> + whether cgroup hierarchy is viewed as flat or hierarchical by the policy.
> + This is how memory controller also has implemented the things.
> +
> Various user visible config options
> ===================================
> CONFIG_BLK_CGROUP


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