Messages in this thread | | | Date | Wed, 29 Mar 2023 13:43:32 -0400 | Subject | Re: CLONE_INTO_CGROUP probably needs to call controller attach handlers | From | Waiman Long <> |
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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
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