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

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Message-ID: <CALvZod62OZVQ7sYKY5V4685eo3RC8esXNyK87JG9VLX3bjeKpw@mail.gmail.com>
Date: Fri, 2 Feb 2024 20:34:33 -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 8:23 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Fri, Feb 2, 2024 at 8:13 PM Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > 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?
>
> Yes. I am trying to ensure that stats_updates sits at the beginning of
> a cacheline to ensure they all fit in one cacheline. Is this
> implicitly guaranteed somehow?
>
> >
> > I would say just remove the CACHELINE_PADDING() from the previous
> > patch and we are good.
>
> IIUC, without CACHELINE_PADDING(), they may end up on different cache
> lines, depending on the size of the arrays before them in the struct
> (which depends on several configs). Am I misunderstanding?
>

Yeah keeping them at the start will be better. Move
____cacheline_aligned to the end of the struct definition like:

struct memcg_vmstats_percpu {
..
} ____cacheline_aligned;


> >
> > 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.
>
> For now, I am just looking for something simple to fix the struct size
> proliferation for v6.8, but this would be interesting to see. I wonder
> how __cacheline_group_end() works since the end is decided already by
> __cacheline_group_begin() and the cacheline size.