Re: [RFC] blk-cgroup: Allow creation of hierarchical cgroups

[Gui Jianfeng]

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@xxxxxxxxxx>

Hi Vivek,

This patch looks good to me.

Reviewed-by: Gui Jianfeng <guijianfeng@xxxxxxxxxxxxxx>

> ---
>  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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