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

[Michal Hocko <mhocko@suse.cz>]

On Tue 19-05-15 14:13:21, Michal Hocko wrote:
> On Mon 18-05-15 15:49:51, Tejun Heo wrote:
> > If move_charge flag is set, memcg tries to move memory charges to the
> > destnation css.  The current implementation migrates memory whenever
> > any thread of a process is migrated making the behavior somewhat
> > arbitrary. 
> 
> This is not true. We have:
>                 mm = get_task_mm(p);
>                 if (!mm)
>                         return 0;
>                 /* We move charges only when we move a owner of the mm */
>                 if (mm->owner == p) {
> 
> So we are ignoring threads which are not owner of the mm struct and that
> should be the thread group leader AFAICS.
> 
> mm_update_next_owner is rather complex (maybe too much and it would
> deserve some attention) so there might really be some corner cases but
> the whole memcg code relies on mm->owner rather than thread group leader
> so I would keep the same logic here.
> 
> > Let's tie memory operations to the threadgroup leader so
> > that memory is migrated only when the leader is migrated.
> 
> This would lead to another strange behavior when the group leader is not
> owner (if that is possible at all) and the memory wouldn't get migrated
> at all.
> 
> I am trying to wrap my head around mm_update_next_owner and maybe we can
> change it to use the thread group leader but this needs more thinking...

OK, I guess I see the reason now. CLONE_VM doesn't imply CLONE_THREAD
so we can have a separate task (with it's own group leader) which
handles its own signals while it still shares the address space. So we
cannot really use the group leader here.
-- 
Michal Hocko
SUSE Labs

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