Re: [External] Re: [PATCH] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Dec 7, 2020 at 8:36 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Sun 06-12-20 16:56:39, Muchun Song wrote:
> > The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
> > of lruvec_stat_cpu is enough. And introduce struct per_cpu_lruvec_stat
> > to optimize memory usage.
>
> How much savings are we talking about here? I am not deeply familiar
> with the pcp allocator but can it compact smaller data types much
> better?

It is a percpu struct. The size of struct lruvec_stat is 304(tested on the
linux-5.5). So we can save 304 / 2 * nproc bytes per memcg where nproc
is the number of the possible CPU. If we have n memory cgroup in the
system. Finally, we can save (152 * nproc * n) bytes. In some configurations,
nproc here may be 512. And if we have a lot of dying cgroup. The n can be
100, 000 (I once saw it on my server).

It can not be smaller. Because the vmstat threshold is 32, and we have
some vmstat counters whose unit is byte. So the max bytes is 32 * 4k
which is 131072. The s16 can't hold these. Thanks.

>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  include/linux/memcontrol.h | 6 +++++-
> >  mm/memcontrol.c            | 2 +-
> >  2 files changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index f9a496c4eac7..34cf119976b1 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -92,6 +92,10 @@ struct lruvec_stat {
> >       long count[NR_VM_NODE_STAT_ITEMS];
> >  };
> >
> > +struct per_cpu_lruvec_stat {
> > +     s32 count[NR_VM_NODE_STAT_ITEMS];
> > +};
> > +
> >  /*
> >   * Bitmap of shrinker::id corresponding to memcg-aware shrinkers,
> >   * which have elements charged to this memcg.
> > @@ -111,7 +115,7 @@ struct mem_cgroup_per_node {
> >       struct lruvec_stat __percpu *lruvec_stat_local;
> >
> >       /* Subtree VM stats (batched updates) */
> > -     struct lruvec_stat __percpu *lruvec_stat_cpu;
> > +     struct per_cpu_lruvec_stat __percpu *lruvec_stat_cpu;
> >       atomic_long_t           lruvec_stat[NR_VM_NODE_STAT_ITEMS];
> >
> >       unsigned long           lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 49fbcf003bf5..c874ea37b05d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5184,7 +5184,7 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
> >               return 1;
> >       }
> >
> > -     pn->lruvec_stat_cpu = alloc_percpu_gfp(struct lruvec_stat,
> > +     pn->lruvec_stat_cpu = alloc_percpu_gfp(struct per_cpu_lruvec_stat,
> >                                              GFP_KERNEL_ACCOUNT);
> >       if (!pn->lruvec_stat_cpu) {
> >               free_percpu(pn->lruvec_stat_local);
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs



-- 
Yours,
Muchun