Re: [PATCH v2 1/7] memcg: reduce memory size of mem_cgroup_events_index

[Yosry Ahmed]

On Fri, Apr 26, 2024 at 5:37 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
>
> mem_cgroup_events_index is a translation table to get the right index of
> the memcg relevant entry for the general vm_event_item. At the moment,
> it is defined as integer array. However on a typical system the max
> entry of vm_event_item (NR_VM_EVENT_ITEMS) is 113, so we don't need to
> use int as storage type of the array. For now just use int8_t as type
> and add a BUILD_BUG_ON() and will switch to short once NR_VM_EVENT_ITEMS
> touches 127.

Any reason not to use uint8_t (or simply u8) and U8_MAX (instead of
the hardcoded 127)?

>
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> ---
>  mm/memcontrol.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 602ad5faad4d..53769d06053f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -607,11 +607,14 @@ static const unsigned int memcg_vm_event_stat[] = {
>  };
>
>  #define NR_MEMCG_EVENTS ARRAY_SIZE(memcg_vm_event_stat)
> -static int mem_cgroup_events_index[NR_VM_EVENT_ITEMS] __read_mostly;
> +static int8_t mem_cgroup_events_index[NR_VM_EVENT_ITEMS] __read_mostly;
>
>  static void init_memcg_events(void)
>  {
> -       int i;
> +       int8_t i;
> +
> +       /* Switch to short once this failure occurs. */
> +       BUILD_BUG_ON(NR_VM_EVENT_ITEMS >= 127 /* INT8_MAX */);
>
>         for (i = 0; i < NR_MEMCG_EVENTS; ++i)
>                 mem_cgroup_events_index[memcg_vm_event_stat[i]] = i + 1;
> --
> 2.43.0
>
>