[Devel] Re: [RFC] cgroup basic comounting

[Li Zefan]

Glauber Costa wrote:
> Turns out that most of the infrastructure we need to put two controllers in the
> same hierarchy is by far already into place. All we need to do is not failing
> when we specify two of them.
> 

You don't need to change anything to mount with 2 cgroup subsystems:

	# mount -t cgroup -o cpu,cpuacct xxx /mnt

But you may want to revise and make use of the subsys->bind() callback, which
is called at mount/remount/umount when we attach/remove a controller to/from
a hierarchy. It's the place you can check if two controllers are going to
be comounted/seperated.

> With this, we can effectively guarantee that by comounting cpu and cpuacct,
> we'll have the same set of tasks, therefore allowing us to use cpu cgroup data
> to fill in the usage fields in cpuacct.
> 
> I decided not to stabilish any dependency between cgroups as Li previously did:
> cgroups may or may not be comounted, and any of them can be combined (I don't
> see a reason to prevent any combination).
> 
> After testing and some trials, I could verify that the current mount behavior
> plays well under the plans, so I didn't change it. That is:
> 
>  * If subsystems A and B aren't mounted, we can comount them.
>  * If subsystem A is mounted, but B is not:
>    * we can comount them if A has no children,
>    * we fail otherwise
>  * If subsystems A and B are comounted at a location, we can't
>    mount any of them separately at another point. We do can mount
>    them together.
>  * If subsystems A and B are comounted at a location,
>    * we can comount a third subsystem C, if they have no children
>    * we fail otherwise
> 
> Paul,
> 
> Please let me know if this is tuned with the idea you had in mind.
> If this is okay, I patch that extracts usage from cpu cgroup data
> in case of comount would follow.
> 
> Signed-off-by: Glauber Costa <glommer at parallels.com>
> CC: Paul Turner <pjt at google.com>
> CC: Li Zefan <lizf at cn.fujitsu.com>
> ---
>  kernel/cgroup.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 1fd7867..e894a4f 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1211,9 +1211,9 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
>  			set_bit(i, &opts->subsys_bits);
>  			one_ss = true;
>  
> -			break;
> +			continue;
>  		}
> -		if (i == CGROUP_SUBSYS_COUNT)
> +		if (opts->subsys_bits == 0)
>  			return -ENOENT;
>  	}
>