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

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Date:   Tue, 8 Sep 2020 20:57:50 +0000
From:   "Julius Hemanth Pitti (jpitti)" <jpitti@...co.com>
To:     "shy828301@...il.com" <shy828301@...il.com>
CC:     "vdavydov.dev@...il.com" <vdavydov.dev@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "xe-linux-external(mailer list)" <xe-linux-external@...co.com>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "guro@...com" <guro@...com>
Subject: Re: [PATCH v2] mm: memcg: yield cpu when we fail to charge pages

On Tue, 2020-09-08 at 13:31 -0700, 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, indeed.
I just tested with e3336cab2579, and it solved this problem.
Thanks for pointing it out.

> 
> > 
> > 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
> > 
> >