Re: [RFC PATCH] mm/oom_kill: allow oom kill allocating task for non-global case

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jun 7, 2021 at 11:51 AM Waiman Long <llong@redhat.com> wrote:
>
> On 6/7/21 2:43 PM, Shakeel Butt wrote:
> > On Mon, Jun 7, 2021 at 9:45 AM Waiman Long <llong@redhat.com> wrote:
> >> On 6/7/21 12:31 PM, Aaron Tomlin wrote:
> >>> At the present time, in the context of memcg OOM, even when
> >>> sysctl_oom_kill_allocating_task is enabled/or set, the "allocating"
> >>> task cannot be selected, as a target for the OOM killer.
> >>>
> >>> This patch removes the restriction entirely.
> >>>
> >>> Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
> >>> ---
> >>>    mm/oom_kill.c | 6 +++---
> >>>    1 file changed, 3 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> >>> index eefd3f5fde46..3bae33e2d9c2 100644
> >>> --- a/mm/oom_kill.c
> >>> +++ b/mm/oom_kill.c
> >>> @@ -1089,9 +1089,9 @@ bool out_of_memory(struct oom_control *oc)
> >>>                oc->nodemask = NULL;
> >>>        check_panic_on_oom(oc);
> >>>
> >>> -     if (!is_memcg_oom(oc) && sysctl_oom_kill_allocating_task &&
> >>> -         current->mm && !oom_unkillable_task(current) &&
> >>> -         oom_cpuset_eligible(current, oc) &&
> >>> +     if (sysctl_oom_kill_allocating_task && current->mm &&
> >>> +            !oom_unkillable_task(current) &&
> >>> +            oom_cpuset_eligible(current, oc) &&
> >>>            current->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {
> >>>                get_task_struct(current);
> >>>                oc->chosen = current;
> >> To provide more context for this patch, we are actually seeing that in a
> >> customer report about OOM happened in a container where the dominating
> >> task used up most of the memory and it happened to be the task that
> >> triggered the OOM with the result that no killable process could be
> >> found.
> > Why was there no killable process? What about the process allocating
> > the memory or is this remote memcg charging?
>
> It is because the other processes have a oom_adjust_score of -1000. So
> they are non-killable. Anyway, they don't consume that much memory and
> killing them won't free up that much.
>
> The other process that uses most of the memory is the one that trigger
> the OOM kill in the first place because the memory limit has been
> reached in new memory allocation. Based on the current logic, this
> process cannot be killed at all even if we set the
> oom_kill_allocating_task to 1 if the OOM happens only within the memcg
> context, not in a global OOM situation.

I am not really against the patch but I am still not able to
understand why select_bad_process() was not able to select the current
process. mem_cgroup_scan_tasks() traverses all the processes in the
target memcg hierarchy, so why the current was skipped.

> This patch is to allow this
> process to be killed under this circumstance.
>
> Cheers,
> Longman
>