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    SubjectRe: [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@cisco.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@cisco.com>
    > > Acked-by: Roman Gushchin <guro@fb.com>
    > > ---
    > >
    > > Changes in v2:
    > > - Added comments.
    > > - Added "Acked-by: Roman Gushchin <guro@fb.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

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