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

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Message-ID: <YfgaIcSJhqEOnh65@carbon.dhcp.thefacebook.com>
Date:   Mon, 31 Jan 2022 09:19:29 -0800
From:   Roman Gushchin <guro@...com>
To:     Waiman Long <longman@...hat.com>
CC:     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>,
        <linux-kernel@...r.kernel.org>, <cgroups@...r.kernel.org>,
        <linux-mm@...ck.org>, Shakeel Butt <shakeelb@...gle.com>,
        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 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!