Re: [RFC PATCH v2 1/5] mm, oom: Introduce bpf_oom_evaluate_task

[Chuyi Zhou <zhouchuyi@bytedance.com>]

Hello,
在 2023/8/17 11:22, Alexei Starovoitov 写道:
> On Wed, Aug 16, 2023 at 7:51 PM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> Hello,
>>
>> 在 2023/8/17 10:07, Alexei Starovoitov 写道:
>>> On Thu, Aug 10, 2023 at 1:13 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>>>    static int oom_evaluate_task(struct task_struct *task, void *arg)
>>>>    {
>>>>           struct oom_control *oc = arg;
>>>> @@ -317,6 +339,26 @@ static int oom_evaluate_task(struct task_struct *task, void *arg)
>>>>           if (!is_memcg_oom(oc) && !oom_cpuset_eligible(task, oc))
>>>>                   goto next;
>>>>
>>>> +       /*
>>>> +        * If task is allocating a lot of memory and has been marked to be
>>>> +        * killed first if it triggers an oom, then select it.
>>>> +        */
>>>> +       if (oom_task_origin(task)) {
>>>> +               points = LONG_MAX;
>>>> +               goto select;
>>>> +       }
>>>> +
>>>> +       switch (bpf_oom_evaluate_task(task, oc)) {
>>>> +       case BPF_EVAL_ABORT:
>>>> +               goto abort; /* abort search process */
>>>> +       case BPF_EVAL_NEXT:
>>>> +               goto next; /* ignore the task */
>>>> +       case BPF_EVAL_SELECT:
>>>> +               goto select; /* select the task */
>>>> +       default:
>>>> +               break; /* No BPF policy */
>>>> +       }
>>>> +
>>>
>>> I think forcing bpf prog to look at every task is going to be limiting
>>> long term.
>>> It's more flexible to invoke bpf prog from out_of_memory()
>>> and if it doesn't choose a task then fallback to select_bad_process().
>>> I believe that's what Roman was proposing.
>>> bpf can choose to iterate memcg or it might have some side knowledge
>>> that there are processes that can be set as oc->chosen right away,
>>> so it can skip the iteration.
>>
>> IIUC, We may need some new bpf features if we want to iterating
>> tasks/memcg in BPF, sush as:
>> bpf_for_each_task
>> bpf_for_each_memcg
>> bpf_for_each_task_in_memcg
>> ...
>>
>> It seems we have some work to do first in the BPF side.
>> Will these iterating features be useful in other BPF scenario except OOM
>> Policy?
> 
> Yes.
> Use open coded iterators though.
> Like example in
> https://lore.kernel.org/all/20230810183513.684836-4-davemarchevsky@fb.com/
> 
> bpf_for_each(task_vma, vma, task, 0) { ... }
> will safely iterate vma-s of the task.
> Similarly struct css_task_iter can be hidden inside bpf open coded iterator.
OK. I think the following APIs whould be useful and I am willing to 
start with these in another bpf-next RFC patchset:

1. bpf_for_each(task). Just like for_each_process(p) in kernel to 
itearing all tasks in the system with rcu_read_lock().

2. bpf_for_each(css_task, task, css). It works like 
css_task_iter_{start, next, end} and would be used to iterating 
tasks/threads under a css.

3. bpf_for_each(descendant_css, css, root_css, {PRE, POST}). It works 
like css_next_descendant_{pre, post} to iterating all descendant.

If you have better ideas or any advice, please let me know.
Thanks.