Messages in this thread | | | From | Richard Palethorpe <> | Subject | Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from charging root | Date | Fri, 16 Oct 2020 16:05:21 +0100 |
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Hello,
Richard Palethorpe <rpalethorpe@suse.de> writes:
> Hello Michal, > > Michal Koutný <mkoutny@suse.com> writes: > >> Hello. >> >> On Wed, Oct 14, 2020 at 08:07:49PM +0100, Richard Palethorpe <rpalethorpe@suse.com> wrote: >>> SLAB objects which outlive their memcg are moved to their parent >>> memcg where they may be uncharged. However if they are moved to the >>> root memcg, uncharging will result in negative page counter values as >>> root has no page counters. >> Why do you think those are reparented objects? If those are originally >> charged in a non-root cgroup, then the charge value should be propagated up the >> hierarchy, including root memcg, so if they're later uncharged in root >> after reparenting, it should still break even. (Or did I miss some stock >> imbalance?) > > I traced it and can see they are reparented objects and that the root > groups counters are zero (or negative if I run madvise06 multiple times) > before a drain takes place. I'm guessing this is because the root group > has 'use_hierachy' set to false so that the childs page_counter parents > are set to NULL. However I will check, because I'm not sure about > either.
Yes, it appears that use_hierarchy=0 which is probably because the test mounts cgroup v1, creates a child group within that and does not set use_hierarchy on the root. On v2 root always has use_hierarchy enabled.
> >> >> (But the patch seems justifiable to me as objects (not)charged directly to >> root memcg may be incorrectly uncharged.) >> >> Thanks, >> Michal
I'm don't know if that could happen without reparenting. I suppose if use_hierarchy=1 then actually this patch will result in root being overcharged, so perhaps it should also check for use_hierarchy?
-- Thank you, Richard.
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