linux-kernel - Re: [PATCH v2] mm: memcg: yield cpu when we fail to charge pages

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Date:   Tue, 8 Sep 2020 22:53:10 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     Julius Hemanth Pitti <jpitti@...co.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        xe-linux-external@...co.com
Subject: Re: [PATCH v2] mm: memcg: yield cpu when we fail to charge pages

On Tue 08-09-20 13:31:51, Yang Shi wrote:
> On Tue, Sep 8, 2020 at 1:14 PM Julius Hemanth Pitti <jpitti@...co.com> wrote:
> >
> > For non root CG, in try_charge(), we keep trying
> > to charge until we succeed. On non-preemptive
> > kernel, when we are OOM, this results in holding
> > CPU forever.
> >
> > On SMP systems, this doesn't create a big problem
> > because oom_reaper get a change to kill victim
> > and make some free pages. However on a single-core
> > CPU (or cases where oom_reaper pinned to same CPU
> > where try_charge is executing), oom_reaper shall
> > never get scheduled and we stay in try_charge forever.
> >
> > Steps to repo this on non-smp:
> > 1. mount -t tmpfs none /sys/fs/cgroup
> > 2. mkdir /sys/fs/cgroup/memory
> > 3. mount -t cgroup none /sys/fs/cgroup/memory -o memory
> > 4. mkdir /sys/fs/cgroup/memory/0
> > 5. echo 40M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
> > 6. echo $$ > /sys/fs/cgroup/memory/0/tasks
> > 7. stress -m 5 --vm-bytes 10M --vm-hang 0
> 
> Isn't it the same problem solved by e3336cab2579 ("mm: memcg: fix
> memcg reclaim soft lockup")? It has been in Linus's tree.

Yes it should because it adds a scheduling point regardless
of reclaimability.
 
> > Signed-off-by: Julius Hemanth Pitti <jpitti@...co.com>
> > Acked-by: Roman Gushchin <guro@...com>
> > ---
> >
> > Changes in v2:
> >  - Added comments.
> >  - Added "Acked-by: Roman Gushchin <guro@...com>".
> > ---
> >  mm/memcontrol.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index cfa6cbad21d5..4f293bf8c7ed 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2745,6 +2745,15 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >         if (fatal_signal_pending(current))
> >                 goto force;
> >
> > +       /*
> > +        * We failed to charge even after retries, give oom_reaper or
> > +        * other process a change to make some free pages.
> > +        *
> > +        * On non-preemptive, Non-SMP system, this is critical, else
> > +        * we keep retrying with no success, forever.
> > +        */
> > +       cond_resched();
> > +
> >         /*
> >          * keep retrying as long as the memcg oom killer is able to make
> >          * a forward progress or bypass the charge if the oom killer
> > --
> > 2.17.1
> >
> >

-- 
Michal Hocko
SUSE Labs