Re: [PATCH v1 10/14] bpf: selftests: bpf OOM handler test

[Roman Gushchin <roman.gushchin@linux.dev>]

Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:

> On Mon, 18 Aug 2025 at 19:02, Roman Gushchin <roman.gushchin@linux.dev> wrote:
>>
>> Implement a pseudo-realistic test for the OOM handling
>> functionality.
>>
>> The OOM handling policy which is implemented in bpf is to
>> kill all tasks belonging to the biggest leaf cgroup, which
>> doesn't contain unkillable tasks (tasks with oom_score_adj
>> set to -1000). Pagecache size is excluded from the accounting.
>>
>> The test creates a hierarchy of memory cgroups, causes an
>> OOM at the top level, checks that the expected process will be
>> killed and checks memcg's oom statistics.
>>
>> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
>> ---
>>  [...]
>> +
>> +/*
>> + * Find the largest leaf cgroup (ignoring page cache) without unkillable tasks
>> + * and kill all belonging tasks.
>> + */
>> +SEC("struct_ops.s/handle_out_of_memory")
>> +int BPF_PROG(test_out_of_memory, struct oom_control *oc)
>> +{
>> +       struct task_struct *task;
>> +       struct mem_cgroup *root_memcg = oc->memcg;
>> +       struct mem_cgroup *memcg, *victim = NULL;
>> +       struct cgroup_subsys_state *css_pos;
>> +       unsigned long usage, max_usage = 0;
>> +       unsigned long pagecache = 0;
>> +       int ret = 0;
>> +
>> +       if (root_memcg)
>> +               root_memcg = bpf_get_mem_cgroup(&root_memcg->css);
>> +       else
>> +               root_memcg = bpf_get_root_mem_cgroup();
>> +
>> +       if (!root_memcg)
>> +               return 0;
>> +
>> +       bpf_rcu_read_lock();
>> +       bpf_for_each(css, css_pos, &root_memcg->css, BPF_CGROUP_ITER_DESCENDANTS_POST) {
>> +               if (css_pos->cgroup->nr_descendants + css_pos->cgroup->nr_dying_descendants)
>> +                       continue;
>> +
>> +               memcg = bpf_get_mem_cgroup(css_pos);
>> +               if (!memcg)
>> +                       continue;
>> +
>> +               usage = bpf_mem_cgroup_usage(memcg);
>> +               pagecache = bpf_mem_cgroup_page_state(memcg, NR_FILE_PAGES);
>> +
>> +               if (usage > pagecache)
>> +                       usage -= pagecache;
>> +               else
>> +                       usage = 0;
>> +
>> +               if ((usage > max_usage) && mem_cgroup_killable(memcg)) {
>> +                       max_usage = usage;
>> +                       if (victim)
>> +                               bpf_put_mem_cgroup(victim);
>> +                       victim = bpf_get_mem_cgroup(&memcg->css);
>> +               }
>> +
>> +               bpf_put_mem_cgroup(memcg);
>> +       }
>> +       bpf_rcu_read_unlock();
>> +
>> +       if (!victim)
>> +               goto exit;
>> +
>> +       bpf_for_each(css_task, task, &victim->css, CSS_TASK_ITER_PROCS) {
>> +               struct task_struct *t = bpf_task_acquire(task);
>> +
>> +               if (t) {
>> +                       if (!bpf_task_is_oom_victim(task))
>> +                               bpf_oom_kill_process(oc, task, "bpf oom test");
>
> Is there a scenario where we want to invoke bpf_oom_kill_process when
> the task is not an oom victim?

Not really, but...

> Would it be better to subsume this check in the kfunc itself?

bpf_task_is_oom_victim() is useful by itself, because if we see
a task which is about to be killed, we can likely simple bail out.
Let me adjust the test to reflect it.