Messages in this thread | | | Date | Fri, 31 Mar 2023 14:27:26 -0400 | Subject | Re: CLONE_INTO_CGROUP probably needs to call controller attach handlers | From | Waiman Long <> |
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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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