Re: [PATCH 6/8] mm: memcontrol: switch to rstat

[Johannes Weiner <hannes@cmpxchg.org>]

On Sun, Feb 07, 2021 at 06:19:04PM -0800, Shakeel Butt wrote:
> On Fri, Feb 5, 2021 at 10:28 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > Replace the memory controller's custom hierarchical stats code with
> > the generic rstat infrastructure provided by the cgroup core.
> >
> > The current implementation does batched upward propagation from the
> > write side (i.e. as stats change). The per-cpu batches introduce an
> > error, which is multiplied by the number of subgroups in a tree. In
> > systems with many CPUs and sizable cgroup trees, the error can be
> > large enough to confuse users (e.g. 32 batch pages * 32 CPUs * 32
> > subgroups results in an error of up to 128M per stat item). This can
> > entirely swallow allocation bursts inside a workload that the user is
> > expecting to see reflected in the statistics.
> >
> > In the past, we've done read-side aggregation, where a memory.stat
> > read would have to walk the entire subtree and add up per-cpu
> > counts. This became problematic with lazily-freed cgroups: we could
> > have large subtrees where most cgroups were entirely idle. Hence the
> > switch to change-driven upward propagation. Unfortunately, it needed
> > to trade accuracy for speed due to the write side being so hot.
> >
> > Rstat combines the best of both worlds: from the write side, it
> > cheaply maintains a queue of cgroups that have pending changes, so
> > that the read side can do selective tree aggregation. This way the
> > reported stats will always be precise and recent as can be, while the
> > aggregation can skip over potentially large numbers of idle cgroups.
> >
> > The way rstat works is that it implements a tree for tracking cgroups
> > with pending local changes, as well as a flush function that walks the
> > tree upwards. The controller then drives this by 1) telling rstat when
> > a local cgroup stat changes (e.g. mod_memcg_state) and 2) when a flush
> > is required to get uptodate hierarchy stats for a given subtree
> > (e.g. when memory.stat is read). The controller also provides a flush
> > callback that is called during the rstat flush walk for each cgroup
> > and aggregates its local per-cpu counters and propagates them upwards.
> >
> > This adds a second vmstats to struct mem_cgroup (MEMCG_NR_STAT +
> > NR_VM_EVENT_ITEMS) to track pending subtree deltas during upward
> > aggregation. It removes 3 words from the per-cpu data. It eliminates
> > memcg_exact_page_state(), since memcg_page_state() is now exact.
> 
> Only if cgroup_rstat_flush() has been called before memcg_page_state(), right?

Yes, correct.

> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > Reviewed-by: Roman Gushchin <guro@fb.com>
> > Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Overall the patch looks good to me with a couple of nits/queries below.
> 
> Reviewed-by: Shakeel Butt <shakeelb@google.com>

Thanks!

> > @@ -1383,8 +1388,16 @@ void count_memcg_event_mm(struct mm_struct *mm, enum vm_event_item idx)
> >  {
> >  }
> >
> > -static inline void lruvec_memcg_debug(struct lruvec *lruvec, struct page *page)
> > +static inline void mem_cgroup_split_huge_fixup(struct page *head)
> > +{
> > +}
> > +
> > +static inline
> > +unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
> > +                                           gfp_t gfp_mask,
> > +                                           unsigned long *total_scanned)
> >  {
> > +       return 0;
> 
> Any technical reason to move around mem_cgroup_soft_limit_reclaim(),
> mem_cgroup_split_huge_fixup() and lruvec_memcg_debug() or just
> aesthetics?

Yeah, just a while-at-it cleanup. It seemed too minor to justify a
separate patch.

> >  #endif /* CONFIG_MEMCG */
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 2f97cb4cef6d..5dc0bd53b64a 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -757,6 +757,11 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz)
> >         return mz;
> >  }
> >
> > +static void memcg_flush_vmstats(struct mem_cgroup *memcg)
> > +{
> > +       cgroup_rstat_flush(memcg->css.cgroup);
> > +}
> 
> cgroup_rstat_flush() has one line wrapper but cgroup_rstat_updated()
> does not, any reason?

cgroup_rstat_flush() seemed a bit low-level to sprinkle around the
code base. Especially with cgroup_rstat_updated() encapsulated by the
mod_memcg_state() layer, a reader of such a callsite might not easily
understand what rstat even is and when and why it needs to be called.

> > @@ -3618,6 +3569,8 @@ static unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
> >  {
> >         unsigned long val;
> >
> > +       memcg_flush_vmstats(memcg);
> > +
> >         if (mem_cgroup_is_root(memcg)) {
> 
> I think memcg_flush_vmstats(memcg) should be here.

Good catch! I'll fix that in the next revision.

Thanks Shakeel