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

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Date:   Wed, 16 Aug 2023 19:07:10 -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@...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 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.