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
