linux-kernel - Re: [PATCH v3 1/8] memcg: reduce memory size of mem_cgroup_events

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Date: Tue, 30 Apr 2024 01:33:38 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, "T . J . Mercier" <tjmercier@...gle.com>, kernel-team@...a.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/8] memcg: reduce memory size of mem_cgroup_events_index

On Mon, Apr 29, 2024 at 11:06 PM Shakeel Butt <shakeel.butt@...ux.dev> 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.
>
> Another benefit of this change is that the translation table fits in 2
> cachelines while previously it would require 8 cachelines (assuming 64
> bytes cachesline).
>
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> Reviewed-by: Roman Gushchin <roman.gushchin@...ux.dev>

Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>

> ---
> Changes since v2:
> - Used S8_MAX instead of 127
> - Update commit message based on Yosry's feedback.
>
>  mm/memcontrol.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 602ad5faad4d..c146187cda9c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -607,11 +607,13 @@ 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;
> +
> +       BUILD_BUG_ON(NR_VM_EVENT_ITEMS >= S8_MAX);
>
>         for (i = 0; i < NR_MEMCG_EVENTS; ++i)
>                 mem_cgroup_events_index[memcg_vm_event_stat[i]] = i + 1;
> --
> 2.43.0
>