Re: [External] Re: [PATCH] mm: memcontrol: fix missing wakeup oom task

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Feb 5, 2021 at 4:24 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Fri 05-02-21 14:23:10, Muchun Song wrote:
> > We call memcg_oom_recover() in the uncharge_batch() to wakeup OOM task
> > when page uncharged, but for the slab pages, we do not do this when page
> > uncharged.
>
> How does the patch deal with this?

When we uncharge a slab page via __memcg_kmem_uncharge,
actually, this path forgets to do this for us compared to
uncharge_batch(). Right?

>
> > When we drain per cpu stock, we also should do this.
>
> Can we have anything the per-cpu stock while entering the OOM path. IIRC
> we do drain all cpus before entering oom path.

You are right. I did not notice this. Thank you.

>
> > The memcg_oom_recover() is small, so make it inline.
>
> Does this lead to any code generation improvements? I would expect
> compiler to be clever enough to inline static functions if that pays
> off. If yes make this a patch on its own.

I have disassembled the code, I see memcg_oom_recover is not
inline. Maybe because memcg_oom_recover has a lot of callers.
Just guess.

(gdb) disassemble uncharge_batch
 [...]
 0xffffffff81341c73 <+227>: callq  0xffffffff8133c420 <page_counter_uncharge>
 0xffffffff81341c78 <+232>: jmpq   0xffffffff81341bc0 <uncharge_batch+48>
 0xffffffff81341c7d <+237>: callq  0xffffffff8133e2c0 <memcg_oom_recover>

>
> > And the parameter
> > of memcg cannot be NULL, so remove the check.
>
> 2bd9bb206b338 has introduced the check without any explanation
> whatsoever. I indeed do not see any potential path to provide a NULL
> memcg here. This is an internal function so the check is unnecessary
> indeed. Please make it a patch on its own.

OK. Will do this. Thanks.

>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  mm/memcontrol.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 8c035846c7a4..8569f4dbea2a 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1925,7 +1925,7 @@ static int memcg_oom_wake_function(wait_queue_entry_t *wait,
> >       return autoremove_wake_function(wait, mode, sync, arg);
> >  }
> >
> > -static void memcg_oom_recover(struct mem_cgroup *memcg)
> > +static inline void memcg_oom_recover(struct mem_cgroup *memcg)
> >  {
> >       /*
> >        * For the following lockless ->under_oom test, the only required
> > @@ -1935,7 +1935,7 @@ static void memcg_oom_recover(struct mem_cgroup *memcg)
> >        * achieved by invoking mem_cgroup_mark_under_oom() before
> >        * triggering notification.
> >        */
> > -     if (memcg && memcg->under_oom)
> > +     if (memcg->under_oom)
> >               __wake_up(&memcg_oom_waitq, TASK_NORMAL, 0, memcg);
> >  }
> >
> > @@ -2313,6 +2313,7 @@ static void drain_stock(struct memcg_stock_pcp *stock)
> >               page_counter_uncharge(&old->memory, stock->nr_pages);
> >               if (do_memsw_account())
> >                       page_counter_uncharge(&old->memsw, stock->nr_pages);
> > +             memcg_oom_recover(old);
> >               stock->nr_pages = 0;
> >       }
> >
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs