linux-kernel - Re: [PATCH 1/3] mm, memcg: Don't put offlined memcg into local stock

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Date:   Mon, 31 Jan 2022 10:00:24 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Roman Gushchin <guro@...com>, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH 1/3] mm, memcg: Don't put offlined memcg into local stock

On Mon, Jan 31, 2022 at 9:25 AM Waiman Long <longman@...hat.com> wrote:
>
> On 1/31/22 12:19, Roman Gushchin wrote:
> > On Mon, Jan 31, 2022 at 12:15:19PM -0500, Waiman Long wrote:
> >> On 1/31/22 12:09, Waiman Long wrote:
> >>> On 1/31/22 12:01, Roman Gushchin wrote:
> >>>> On Sun, Jan 30, 2022 at 10:55:56PM -0500, Waiman Long wrote:
> >>>>> On 10/1/21 19:51, Roman Gushchin wrote:
> >>>>>> On Fri, Oct 01, 2021 at 03:09:36PM -0400, Waiman Long wrote:
> >>>>>>> When freeing a page associated with an offlined memcg, refill_stock()
> >>>>>>> will put it into local stock delaying its demise until another memcg
> >>>>>>> comes in to take its place in the stock. To avoid that, we now check
> >>>>>>> for offlined memcg and go directly in this case to the slowpath for
> >>>>>>> the uncharge via the repurposed cancel_charge() function.
> >>>>>> Hi Waiman!
> >>>>>>
> >>>>>> I'm afraid it can make a cleanup of a dying cgroup slower: for every
> >>>>>> released page we'll potentially traverse the whole cgroup tree and
> >>>>>> decrease atomic page counters.
> >>>>>>
> >>>>>> I'm not sure I understand the benefits we get from this change which
> >>>>>> do justify the slowdown on the cleanup path.
> >>>>>>
> >>>>>> Thanks!
> >>>>> I was notified of a lockdep splat that this patch may help to prevent.
> >>>> Would you mind to test this patch:
> >>>> https://www.spinics.net/lists/cgroups/msg31244.html  ?
> >>>>
> >>>> It should address this dependency.
> >>> Thanks for the pointer. I believe that your patch should be able to
> >>> address this circular locking dependency.
> >>>
> >>> Feel free to add my
> >>>
> >>> Reviewed-by: Waiman Long <longman@...hat.com>
> >> BTW, have you posted it to lkml? If not, would you mind doing so?
> > Not yet.
> >
> > I was waiting for Alexander to confirm that it resolves the originally reported
> > issue. I just pinged him, will wait for tomorrow and post the patch in any case.
> >
> > Thanks!
>
> I see. This is not a problem that is easily reproducible. You need to
> hit the right timing for the lockdep splat to appear.

I agree here. The patch on its own has merits as it is reducing
dependency on an unrelated lock.