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From:   Waiman Long <llong@redhat.com>
Subject: Re: [RFC PATCH] mm/oom_kill: allow oom kill allocating task for
 non-global case
To:     Shakeel Butt <shakeelb@google.com>, Waiman Long <llong@redhat.com>
Cc:     Aaron Tomlin <atomlin@redhat.com>, Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.com>,
        LKML <linux-kernel@vger.kernel.org>
References: <20210607163103.632681-1-atomlin@redhat.com>
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 <dc7f54eb-933e-5bbb-7959-815dfbfcc836@redhat.com>
 <CALvZod71SyjvC7sRtkD7dSEeAbYH_ynvO_hDS=tY6KCt-kRJkQ@mail.gmail.com>
Message-ID: <b5b03f9f-0481-e59f-b2f2-5968f668dbac@redhat.com>
Date:   Mon, 7 Jun 2021 16:07:14 -0400
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On 6/7/21 3:04 PM, Shakeel Butt wrote:
> 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.

Yes, you are right. Probably there is some problem with reaping so that 
the MMF_OOM_SKIP bit gets set. I don't have the answer yet.

Regards,
Longman