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

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Date:   Fri, 5 Feb 2021 19:04:19 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.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, Feb 5, 2021 at 6:21 PM Michal Hocko <mhocko@...e.com> wrote:
>
> 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?

Sorry, the subject title may not be suitable. IIUC, memcg_oom_recover
aims to wake up the OOM task when we uncharge the page.
I see uncharge_batch always do this. I am confused why
__memcg_kmem_uncharge does not. Both paths do the same
thing (uncharge pages). So actually, this patch want to keep
the two paths consistent. Thanks.

>
> > > > 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?

I just think memcg_oom_recover is very small, inline maybe
a good choice. Maybe I am wrong.

> --
> Michal Hocko
> SUSE Labs