Re: [RFC] memcg/cgroup: do not fail fail on pre_destroy callbacks

From: Kamezawa Hiroyuki
Date: Wed Oct 17 2012 - 20:30:31 EST

(2012/10/17 22:30), Michal Hocko wrote:
> Hi,
> memcg is the only controller which might fail in its pre_destroy
> callback which makes the cgroup core more complicated for no good
> reason. This is an attempt to change this unfortunate state.
> I am sending this a RFC because I would like to hear back whether the
> approach is correct. I thought that the changes would be more invasive
> but it seems that the current code was mostly prepared for this and it
> needs just some small tweaks (so I might be missing something important
> here).
> The first two patches are just clean ups. They could be merged even
> without the rest.
> The real change, although the code is not changed that much, is the 3rd
> patch. It changes the way how we handle mem_cgroup_move_parent failures.
> We have to realize that all those failures are *temporal*. Because we
> are either racing with the page removal or the page is temporarily off
> the LRU because of migration resp. global reclaim. As a result we do
> not fail mem_cgroup_force_empty_list if the page cannot be moved to the
> parent and rather retry until the LRU is empty.
> The 4th patch is for cgroup core. I have moved cgroup_call_pre_destroy
> inside the cgroup_lock which is not very nice because the callbacks
> can take some time. Maybe we can move this call at the very end of the
> function?
> All I need for memcg is that cgroup_call_pre_destroy has been called and
> that no new cgroups can be attached to the group. The cgroup_lock is
> necessary for the later condition but if we move after CGRP_REMOVED flag
> is set then we are safe as well.
> The last two patches are trivial follow ups for the cgroups core change
> because now we know that nobody will interfere with us so we can drop
> those empty && no child condition.
> Comments, thoughts?
> Michal Hocko (6):
> memcg: split mem_cgroup_force_empty into reclaiming and reparenting parts
> memcg: root_cgroup cannot reach mem_cgroup_move_parent
> memcg: Simplify mem_cgroup_force_empty_list error handling
> cgroups: forbid pre_destroy callback to fail
> memcg: make mem_cgroup_reparent_charges non failing
> hugetlb: do not fail in hugetlb_cgroup_pre_destroy
> Cumulative diffstat:
> kernel/cgroup.c | 30 ++++---------
> mm/hugetlb_cgroup.c | 11 ++---
> mm/memcontrol.c | 124 +++++++++++++++++++++++++++------------------------
> 3 files changed, 78 insertions(+), 87 deletions(-)

Thank you very much ! The whole patch seems good to me and I like this approach.


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