Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> writes:
> On Mon, 18 Aug 2025 at 19:02, Roman Gushchin <roman.gushchin@xxxxxxxxx> 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@xxxxxxxxx>
>> ---
>> [...]
>> +
>> +/*
>> + * 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.
