Re: [PATCH 2/3] mm: vmscan: detect file thrashing at the reclaim root

[Joonsoo Kim]

2020ë 2ì 13ì (ë) ìì 3:18, Johannes Weiner <hannes@xxxxxxxxxxx>ëì ìì:
>
> On Wed, Feb 12, 2020 at 07:28:19PM +0900, Joonsoo Kim wrote:
> > Hello, Johannes.
> >
> > When I tested my patchset on v5.5, I found that my patchset doesn't
> > work as intended. I tracked down the issue and this patch would be the
> > reason of unintended work. I don't fully understand the patchset so I
> > could be wrong. Please let me ask some questions.
> >
> > On Thu, Nov 07, 2019 at 12:53:33PM -0800, Johannes Weiner wrote:
> > ...snip...
> > > -static void snapshot_refaults(struct mem_cgroup *root_memcg, pg_data_t *pgdat)
> > > +static void snapshot_refaults(struct mem_cgroup *target_memcg, pg_data_t *pgdat)
> > >  {
> > > -   struct mem_cgroup *memcg;
> > > -
> > > -   memcg = mem_cgroup_iter(root_memcg, NULL, NULL);
> > > -   do {
> > > -           unsigned long refaults;
> > > -           struct lruvec *lruvec;
> > > +   struct lruvec *target_lruvec;
> > > +   unsigned long refaults;
> > >
> > > -           lruvec = mem_cgroup_lruvec(memcg, pgdat);
> > > -           refaults = lruvec_page_state_local(lruvec, WORKINGSET_ACTIVATE);
> > > -           lruvec->refaults = refaults;
> > > -   } while ((memcg = mem_cgroup_iter(root_memcg, memcg, NULL)));
> > > +   target_lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
> > > +   refaults = lruvec_page_state(target_lruvec, WORKINGSET_ACTIVATE);
> > > +   target_lruvec->refaults = refaults;
> >
> > Is it correct to just snapshot the refault for the target memcg? I
> > think that we need to snapshot the refault for all the child memcgs
> > since we have traversed all the child memcgs with the refault count
> > that is aggregration of all the child memcgs. If next reclaim happens
> > from the child memcg, workingset transition that is already considered
> > could be considered again.
>
> Good catch, you're right! We have to update all cgroups in the tree,
> like we used to. However, we need to use lruvec_page_state() instead
> of _local, because we do recursive comparisons in shrink_node()! So
> it's not a clean revert of that hunk.
>
> Does this patch here fix the problem you are seeing?

I found that my problem comes from my mistake.
Sorry for bothering you!

Anyway, following hunk looks correct to me.

Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c82e9831003f..e7431518db13 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2993,12 +2993,17 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>
>  static void snapshot_refaults(struct mem_cgroup *target_memcg, pg_data_t *pgdat)
>  {
> -       struct lruvec *target_lruvec;
> -       unsigned long refaults;
> +       struct mem_cgroup *memcg;
>
> -       target_lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
> -       refaults = lruvec_page_state(target_lruvec, WORKINGSET_ACTIVATE);
> -       target_lruvec->refaults = refaults;
> +       memcg = mem_cgroup_iter(target_memcg, NULL, NULL);
> +       do {
> +               unsigned long refaults;
> +               struct lruvec *lruvec;
> +
> +               lruvec = mem_cgroup_lruvec(memcg, pgdat);
> +               refaults = lruvec_page_state(lruvec, WORKINGSET_ACTIVATE);
> +               lruvec->refaults = refaults;
> +       } while ((memcg = mem_cgroup_iter(target_memcg, memcg, NULL)));
>  }
>
>  /*
>
> > > @@ -277,12 +305,12 @@ void workingset_refault(struct page *page, void *shadow)
> > >      * would be better if the root_mem_cgroup existed in all
> > >      * configurations instead.
> > >      */
> > > -   memcg = mem_cgroup_from_id(memcgid);
> > > -   if (!mem_cgroup_disabled() && !memcg)
> > > +   eviction_memcg = mem_cgroup_from_id(memcgid);
> > > +   if (!mem_cgroup_disabled() && !eviction_memcg)
> > >             goto out;
> > > -   lruvec = mem_cgroup_lruvec(memcg, pgdat);
> > > -   refault = atomic_long_read(&lruvec->inactive_age);
> > > -   active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES);
> > > +   eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
> > > +   refault = atomic_long_read(&eviction_lruvec->inactive_age);
> > > +   active_file = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE);
> >
> > Do we need to use the aggregation LRU count of all the child memcgs?
> > AFAIU, refault here is the aggregation counter of all the related
> > memcgs. Without using the aggregation count for LRU, active_file could
> > be so small than the refault distance and refault cannot happen
> > correctly.
>
> lruvec_page_state() *is* aggregated for all child memcgs (as opposed
> to lruvec_page_state_local()), so that comparison looks correct to me.

Thanks for informing this.
I have checked lruvec_page_state() but not mod_lruvec_state() so cannot
find that counter is the aggregated value.

Thanks.