linux-kernel - Re: [RFC PATCH v2 1/5] mm, oom: Introduce bpf_oom_evaluate

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Date:   Thu, 17 Aug 2023 21:34:33 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Chuyi Zhou <zhouchuyi@...edance.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, wuyun.abel@...edance.com,
        robin.lu@...edance.com, Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH v2 1/5] mm, oom: Introduce bpf_oom_evaluate_task

On Thu, Aug 17, 2023 at 8:30 PM Chuyi Zhou <zhouchuyi@...edance.com> wrote:
>
> Hello,
> 在 2023/8/17 11:22, Alexei Starovoitov 写道:
> > On Wed, Aug 16, 2023 at 7:51 PM Chuyi Zhou <zhouchuyi@...edance.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@...edance.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.

Sounds great. Such 3 new iterators are unrelated to oom discussion and
can be developed/landed in parallel.
They will be useful in other bpf programs.