Re: [PATCH] memcg: make it work on sparse non-0-node systems

Michal Hocko
2019-04-29T13:15:56Z

On Mon 29-04-19 13:55:26, Jiri Slaby wrote:
> On 29. 04. 19, 13:30, Michal Hocko wrote:
> > On Mon 29-04-19 12:59:39, Jiri Slaby wrote:
> > [...]
> >> static inline bool list_lru_memcg_aware(struct list_lru *lru)
> >> {
> >> - /*
> >> - * This needs node 0 to be always present, even
> >> - * in the systems supporting sparse numa ids.
> >> - */
> >> - return !!lru->node[0].memcg_lrus;
> >> + return !!lru->node[first_online_node].memcg_lrus;
> >> }
> >>
> >> static inline struct list_lru_one *
> >
> > How come this doesn't blow up later - e.g. in memcg_destroy_list_lru
> > path which does iterate over all existing nodes thus including the
> > node 0.
>
> If the node is not disabled (i.e. is N_POSSIBLE), lru->node is allocated
> for that node too. It will also have memcg_lrus properly set.
>
> If it is disabled, it will never be iterated.
>
> Well, I could have used first_node. But I am not sure, if the first
> POSSIBLE node is also ONLINE during boot?

I dunno. I would have to think about this much more. The whole
expectation that node 0 is always around is simply broken. But also
list_lru_memcg_aware looks very suspicious. We should have a flag or
something rather than what we have now.

I am still not sure I have completely understood the problem though.
I will try to get to this during the week but Vladimir should be much
better fit to judge here.
--
Michal Hocko
SUSE Labs