Re: [PATCH 3/7] memcg: immigrate charges only when a threadgroup leader is moved

[Michal Hocko <mhocko@suse.cz>]

On Wed 20-05-15 19:53:02, Oleg Nesterov wrote:
> On 05/20, Michal Hocko wrote:
> >
> > So I assume the leader simply waits for its threads to finish and it
> > stays in the sibling list. __unhash_process seems like it does the final
> > cleanup and unlinks the leader from the lists. Which means that
> > mm_update_next_owner never sees !group_leader. Is that correct Oleg?
> 
> Yes, yes, the group leader can't go away until the whole thread-group dies.

OK, then we should have a guarantee that mm->owner is always thread
group leader, right?

> But can't we kill mm->owner somehow?

I would be happy about that. But it is not that simple.

> I mean, turn it into something else,
> ideally into "struct mem_cgroup *" although I doubt this is possible.

Sounds like a good idea but... it duplicates the cgroup tracking into
two places and that asks for troubles. On the other hand we are doing
that already because mm->owner might be in a different cgroup than the
current. However, this is an inherent problem because CLONE_VM doesn't
imply CLONE_THREAD. So in the end it doesn't look much worse IMO.
We will loose the "this task is in charge" aspect and that would
be a user space visible change but I am not sure how much it is a
problem. Maybe somebody is (ab)using this to workaround the restriction
that all threads are in the same cgroup.

>From the implementation POV it even looks easier because we just have to
hook to fork (pin the memcg on dup_mm), to attach to change the memcg 
and to mmput to unpin the memcg.

I will think about that some more.

> It would be nice to kill mm_update_next_owner()/etc, this looks really
> ugly. We only need it for mem_cgroup_from_task(), and it would be much
> more clean to have mem_cgroup_from_mm(struct mm_struct *mm), imho.
> 
> Oleg.
> 

-- 
Michal Hocko
SUSE Labs

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