linux-kernel - Re: [PATCH mm-hotfixes-unstable] mm: memcg: fix struct memcg_vmstats

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Date: Fri, 2 Feb 2024 20:13:24 -0800
From: Shakeel Butt <shakeelb@...gle.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
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>, cgroups@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH mm-hotfixes-unstable] mm: memcg: fix struct
 memcg_vmstats_percpu size and alignment

On Fri, Feb 2, 2024 at 4:34 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> Commit da10d7e140196 ("mm: memcg: optimize parent iteration in
> memcg_rstat_updated()") added two additional pointers to the end of
> struct memcg_vmstats_percpu with CACHELINE_PADDING to put them in a
> separate cacheline. This caused the struct size to increase from 1200 to
> 1280 on my config (80 extra bytes instead of 16).
>
> Upon revisiting, the relevant struct members do not need to be on a
> separate cacheline, they just need to fit in a single one. This is a
> percpu struct, so there shouldn't be any contention on that cacheline
> anyway. Move the members to the beginning of the struct and cachealign
> the first member. Add a comment about the members that need to fit
> together in a cacheline.
>
> The struct size is now 1216 on my config with this change.
>
> Fixes: da10d7e140196 ("mm: memcg: optimize parent iteration in memcg_rstat_updated()")
> Reported-by: Greg Thelen <gthelen@...gle.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
>  mm/memcontrol.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d9ca0fdbe4ab0..09f09f37e397e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -621,6 +621,15 @@ static inline int memcg_events_index(enum vm_event_item idx)
>  }
>
>  struct memcg_vmstats_percpu {
> +       /* Stats updates since the last flush */
> +       unsigned int                    stats_updates ____cacheline_aligned;

Why do you need ____cacheline_aligned here? From what I understand for
the previous patch you want stats_updates, parent and vmstats on the
same cacheline, right?

I would say just remove the CACHELINE_PADDING() from the previous
patch and we are good.

In the followup I plan to add usage of __cacheline_group_begin() and
__cacheline_group_end() usage in memcg code. If you want, take a stab
at it.