linux-kernel - Re: [PATCH v2] memcg: Only record foreign writebacks with dirty pages when memcg is not disabled

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Date:   Tue, 24 Sep 2019 15:46:06 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     akpm@...ux-foundation.org, tj@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] memcg: Only record foreign writebacks with dirty
 pages when memcg is not disabled

On Tue 24-09-19 21:43:52, Baoquan He wrote:
> On 09/24/19 at 03:16pm, Michal Hocko wrote:
> > On Tue 24-09-19 21:04:58, Baoquan He wrote:
> > > On 09/24/19 at 02:27pm, Michal Hocko wrote:
> > > > On Tue 24-09-19 19:11:51, Baoquan He wrote:
> > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > > index f3c15bb07cce..84e3fdb1ccb4 100644
> > > > > --- a/mm/memcontrol.c
> > > > > +++ b/mm/memcontrol.c
> > > > > @@ -4317,6 +4317,9 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
> > > > >  
> > > > >  	trace_track_foreign_dirty(page, wb);
> > > > >  
> > > > > +	if (mem_cgroup_disabled())
> > > > > +		return;
> > > > > +
> > > > 
> > > > This doesn't seem correct. We shouldn't even enter the slowpath with
> > > > memcg disabled AFAIC. The check should be done at mem_cgroup_track_foreign_dirty
> > > > level.
> > > 
> > > You mean the way in v1 patch, right? It's also fine to me.
> > > 
> > > I am worried about the case that memcg is enabled, the checking by
> > > calling mem_cgroup_disabled() will lower efficiency.
> > 
> > This is hidden by a static branch so I wouldn't really be worried about
> > the overhead.
> > 
> > > And it entering
> > > into mem_cgroup_track_foreign_dirty_slowpath() should be a rare event.
> > 
> > But &page->mem_cgroup->css != wb->memcg_css doesn't make any sense when
> > memcg is disabled, right?
> 
> Yeah, I think so. Make it like below?

Or just put it on its own line to make the code more readable.

> @@ -1261,7 +1261,8 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
>  static inline void mem_cgroup_track_foreign_dirty(struct page *page,
>                                                   struct bdi_writeback *wb)
>  {
> -       if (unlikely(&page->mem_cgroup->css != wb->memcg_css))
> +       if (!mem_cgroup_disabled() &&
> +           unlikely(&page->mem_cgroup->css != wb->memcg_css))
>                 mem_cgroup_track_foreign_dirty_slowpath(page, wb);
>  }

-- 
Michal Hocko
SUSE Labs