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

[Alexei Starovoitov]

On Wed, Aug 16, 2023 at 7:51 PM Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx> wrote:
>
> Hello,
>
> 在 2023/8/17 10:07, Alexei Starovoitov 写道:
> > On Thu, Aug 10, 2023 at 1:13 AM Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx> 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@xxxxxx/

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.