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

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Date:   Fri, 5 Feb 2021 11:21:47 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cgroups <cgroups@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH] mm: memcontrol: fix missing wakeup oom
 task

On Fri 05-02-21 17:55:10, Muchun Song wrote:
> On Fri, Feb 5, 2021 at 4:24 PM Michal Hocko <mhocko@...e.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?

Yes this was more more or less clear (still would have been nicer to be
explicit). But you still haven't replied to my question I believe. I
assume you rely on refill_stock doing draining but how does this address
the problem? Is it sufficient to do wakeups in the batched way?

> > > 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>

So does it really help to do the inlining?
-- 
Michal Hocko
SUSE Labs