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

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Message-ID: <CAJD7tkbzaiOWpxPTf6xbNvy=6HXdLbnGQMGgi6Krq9q4j+L-WQ@mail.gmail.com>
Date: Fri, 2 Feb 2024 20:39:49 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Shakeel Butt <shakeelb@...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 8:34 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> 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@...glecom> 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;

Will send a v2 shortly, thanks. I honestly didn't know what the
difference was, but both gave me the same results. Is using
____cacheline_aligned with the first member the same as using it at
the end of the definition?