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

[Yang Shi <shy828301@gmail.com>]

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.

>
> 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
>
>