On Tue, Apr 24, 2018 at 03:24:53PM +0300, Kirill Tkhai wrote:
> >>>>>> +int expand_shrinker_maps(int old_nr, int nr)
> >>>>>> +{
> >>>>>> + int id, size, old_size, node, ret;
> >>>>>> + struct mem_cgroup *memcg;
> >>>>>> +
> >>>>>> + old_size = old_nr / BITS_PER_BYTE;
> >>>>>> + size = nr / BITS_PER_BYTE;
> >>>>>> +
> >>>>>> + down_write(&shrinkers_max_nr_rwsem);
> >>>>>> + for_each_node(node) {
> >>>>>
> >>>>> Iterating over cgroups first, numa nodes second seems like a better idea
> >>>>> to me. I think you should fold for_each_node in memcg_expand_maps.
> >>>>>
> >>>>>> + idr_for_each_entry(&mem_cgroup_idr, memcg, id) {
> >>>>>
> >>>>> Iterating over mem_cgroup_idr looks strange. Why don't you use
> >>>>> for_each_mem_cgroup?
> >>>>
> >>>> We want to allocate shrinkers maps in mem_cgroup_css_alloc(), since
> >>>> mem_cgroup_css_online() mustn't fail (it's a requirement of currently
> >>>> existing design of memcg_cgroup::id).
> >>>>
> >>>> A new memcg is added to parent's list between two of these calls:
> >>>>
> >>>> css_create()
> >>>> ss->css_alloc()
> >>>> list_add_tail_rcu(&css->sibling, &parent_css->children)
> >>>> ss->css_online()
> >>>>
> >>>> for_each_mem_cgroup() does not see allocated, but not linked children.
> >>>
> >>> Why don't we move shrinker map allocation to css_online then?
> >>
> >> Because the design of memcg_cgroup::id prohibits mem_cgroup_css_online() to fail.
> >> This function can't fail.
> >
> > I fail to understand why it is so. Could you please elaborate?
>
> mem_cgroup::id is freed not in mem_cgroup_css_free(), but earlier. It's freed
> between mem_cgroup_css_offline() and mem_cgroup_free(), after the last reference
> is put.
>
> In case of sometimes we want to free it in mem_cgroup_css_free(), this will
> introduce assymmetric in the logic, which makes it more difficult. There is
> already a bug, which I fixed in
>
> "memcg: remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure"
>
> new change will make this code completely not-modular and unreadable.
How is that? AFAIU all we need to do to handle css_online failure
properly is call mem_cgroup_id_remove() from mem_cgroup_css_free().
That's it, as mem_cgroup_id_remove() is already safe to call more
than once for the same cgroup - the first call will free the id
while all subsequent calls will do nothing.
>
> >>
> >> I don't think it will be good to dive into reworking of this stuff for this patchset,
> >> which is really already big. Also, it will be assymmetric to allocate one part of
> >> data in css_alloc(), while another data in css_free(). This breaks cgroup design,
> >> which specially introduces this two function to differ allocation and onlining.
> >> Also, I've just move the allocation to alloc_mem_cgroup_per_node_info() like it was
> >> suggested in comments to v1...
> >
> > Yeah, but (ab)using mem_cgroup_idr for iterating over all allocated
> > memory cgroups looks rather dubious to me...
>
> But we have to iterate over all allocated memory cgroups in any way,
> as all of them must have expanded maps. What is the problem?
> It's rather simple method, and it faster then for_each_mem_cgroup()
> cycle, since it does not have to play with get and put of refcounters.
I don't like this, because mem_cgroup_idr was initially introduced to
avoid depletion of css ids by offline cgroups. We could fix that problem
by extending swap_cgroup to UINT_MAX, in which case mem_cgroup_idr
wouldn't be needed at all. Reusing mem_cgroup_idr for iterating over
allocated cgroups deprives us of the ability to reconsider that design
decision in future, neither does it look natural IMO. Besides, in order
to use mem_cgroup_idr for your purpose, you have to reshuffle the code
of mem_cgroup_css_alloc in a rather contrived way IMO.
I agree that allocating parts of struct mem_cgroup in css_online may
look dubious, but IMHO it's better than inventing a new way to iterate
over cgroups instead of using the iterator provided by cgroup core.
May be, you should ask Tejun which way he thinks is better.
Thanks,
Vladimir
