Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Shakeel Butt <shakeelb@google.com>]

On Tue, Dec 8, 2020 at 1:53 AM Muchun Song <songmuchun@bytedance.com> 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.
>
> The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
> is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
> bytes per-memcg per-node where ncpu is the number of the possible CPU.
> If there are c memory cgroup (include dying cgroup) and n NUMA node in
> the system. Finally, we can save (152 * ncpu * c * n) bytes.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Few nits below:

Reviewed-by: Shakeel Butt <shakeelb@google.com>

> ---
> Changes in v1 -> v2:
>  - Update the commit log to point out how many bytes that we can save.
>
>  include/linux/memcontrol.h |  6 +++++-
>  mm/memcontrol.c            | 10 +++++++++-
>  2 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 3febf64d1b80..290d6ec8535a 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 {

lruvec_stat is also per-cpu, so the name per_cpu_lruvec_stat does not
really tell why it is different from lruvec. Maybe name is
batched_lruvec_stat or something else.

> +       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;

A comment for the above why it still needs to be lruvec_stat.

>
>         /* 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 eec44918d373..da6dc6ca388d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5198,7 +5198,7 @@ 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);
> @@ -7089,6 +7089,14 @@ static int __init mem_cgroup_init(void)
>  {
>         int cpu, node;
>
> +       /*
> +        * Currently s32 type (can refer to struct per_cpu_lruvec_stat) is
> +        * used for per-memcg-per-cpu caching of per-node statistics. In order
> +        * to work fine, we should make sure that the overfill threshold can't
> +        * exceed S32_MAX / PAGE_SIZE.
> +        */
> +       BUILD_BUG_ON(MEMCG_CHARGE_BATCH > S32_MAX / PAGE_SIZE);
> +
>         cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
>                                   memcg_hotplug_cpu_dead);
>
> --
> 2.11.0
>