Re: CLONE_INTO_CGROUP probably needs to call controller attach handlers

[Waiman Long <longman@redhat.com>]

On 3/29/23 10:52, Johannes Weiner wrote:
> On Tue, Mar 28, 2023 at 10:48:49PM -0400, Waiman Long wrote:
>> On 3/28/23 21:30, Waiman Long wrote:
>>> On 3/28/23 11:39, Christian Brauner wrote:
>>>> Hey,
>>>>
>>>> Giuseppe reported that the the affinity mask isn't updated when a
>>>> process is spawned directly into the target cgroup via
>>>> CLONE_INTO_CGROUP. However, migrating a process will cause the affinity
>>>> mask to be updated (see the repro at [1].
>>>>
>>>> I took a quick look and the issue seems to be that we don't call the
>>>> various attach handlers during CLONE_INTO_CGROUP whereas we do for
>>>> migration. So the solution seems to roughly be that we need to call the
>>>> various attach handlers during CLONE_INTO_CGROUP as well when the
>>>> parent's cgroups is different from the child cgroup. I think we need to
>>>> call all of them, can, cancel and attach.
>>>>
>>>> The plumbing here might be a bit intricate since the arguments that the
>>>> fork handlers take are different from the attach handlers.
>>>>
>>>> Christian
>>>>
>>>> [1]: https://paste.centos.org/view/f434fa1a
>>>>
>>> I saw that the current cgroup code already have the can_fork, fork and
>>> cancel_fork callbacks. Unfortunately such callbacks are not defined for
>>> cpuset yet. That is why the cpu affinity isn't correctly updated. I can
>>> post a patch to add those callback functions to cpuset which should then
>>> able to correctly address this issue.
>> Looking further into this issue, I am thinking that forking into a cgroup
>> should be equivalent to write the child pid into the "cgroup.threads" file
>> of the target cgroup. By taking this route, all the existing can_attach,
>> attach and cancel_attach methods can be used. I believe the original fork
>> method is for the limited use case of forking into the same cgroup. So right
>> now, only the pids controller has the fork methods. Otherwise, we will have
>> to modify a number of different controllers to add the necessary fork
>> methods. They will be somewhat similar to the existing attach methods and so
>> it will be a lot of duplication. What do you think about this idea?
> That's what I thought at first too, but then I had some doubts.
>
> The callback is called 'attach', but it's historically implemented
> when moving an established task between two cgroups. Many controllers
> use it to move state between groups (memcg, pids, cpuset). So in
> practice it isn't the natural fit that its name would suggest, and it
> would require reworking those controllers to handle both scenarios:
> moving tasks between groups, and new tasks attaching to a cgroup.
>
> Now I'm thinking it probably makes more sense to keep using attach for
> moving between groups, and fork for being born into a cgroup. That's
> what the pid controller does, and it handles CLONE_INTO_CGROUP fine.
>
> There is naturally some overlap between the two operations. But it
> seems cleaner to me to use common helpers for that, as opposed to
> having both attach and fork callbacks handling forks.

I was thinking along the line of using common helpers for doing fork and 
attach. However, the expected method function prototypes are quite 
different. For example,

int (*can_attach)(struct cgroup_taskset *tset);
int (*can_fork)(struct task_struct *task, css_set *cset);

We need to make them more similar before we can use common helpers. I 
can take a look at that.

Thanks,
Longman