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    SubjectRe: CLONE_INTO_CGROUP probably needs to call controller attach handlers
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    On 3/29/23 03:19, Christian Brauner 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?
    > The overall plan sounds good to me. I have one comment and question
    > about making this equivalent to a write of the child pid into the
    > cgroup.threads file.
    >
    > The paragraph above seems to imply that CLONE_INTO_CGROUP currently
    > isn't equivalent to a write to cgroup.threads. But it's not that
    > straightforward. CLONE_INTO_CGROUP needs to handle both threads and
    > threadgroups aka being or-ed with CLONE_THREAD or not. It does that in
    > cgroup_css_set_fork() when calling
    > cgroup_attach_permissions([...] !(kargs->flags & CLONE_THREAD), [...]).
    >
    > What it's missing is calling the relevant handlers that would be
    > executed in the migration path. They might be different between the
    > CLONE_THREAD and !CLONE_THREAD case. But the crux remains that
    > CLONE_INTO_CGROUP needs to handle both cases.
    >
    > So afaict, what you're proposing is equivalent to what I sketched in the
    > initial mail? Or is there something else you mean by making this
    > equivalent to cgroup.threads that goes beyond adding the missing
    > handlers? Just trying to make sure we're not accidently changing
    > semantics.

    It turns out that cpuset does have a cpuset_fork() method defined and so
    is the legacy freezer even though they don't have a can_fork method. So
    the simplest way to fix this problem is to extend the existing
    cpuset_fork() method to handle the CLONE_INTO_CGROUP case. I have sent
    out an upstream patch to fix that issue.

    Cheers,
    Longman

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