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SubjectRe: [RFC] cgroup basic comounting
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@parallels.com>
> CC: Paul Turner <pjt@google.com>
> CC: Li Zefan <lizf@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;
> }
>


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