Re: [PATCH bpf-next v2 8/8] bpf: add a selftest for cgroup hierarchical stats collection

Yonghong Song
2022-07-02T00:55:10Z

On 6/29/22 1:04 AM, Yosry Ahmed wrote:
> On Tue, Jun 28, 2022 at 11:48 PM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> On 6/28/22 5:09 PM, Yosry Ahmed wrote:
>>> On Tue, Jun 28, 2022 at 12:14 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>>>>
>>>> On Mon, Jun 27, 2022 at 11:47 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>>>>>
>>>>> On Mon, Jun 27, 2022 at 11:14 PM Yonghong Song <yhs@fb.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 6/10/22 12:44 PM, Yosry Ahmed wrote:
>>>>>>> Add a selftest that tests the whole workflow for collecting,
>>>>>>> aggregating (flushing), and displaying cgroup hierarchical stats.
>>>>>>>
>>>>>>> TL;DR:
>>>>>>> - Whenever reclaim happens, vmscan_start and vmscan_end update
>>>>>>> per-cgroup percpu readings, and tell rstat which (cgroup, cpu) pairs
>>>>>>> have updates.
>>>>>>> - When userspace tries to read the stats, vmscan_dump calls rstat to flush
>>>>>>> the stats, and outputs the stats in text format to userspace (similar
>>>>>>> to cgroupfs stats).
>>>>>>> - rstat calls vmscan_flush once for every (cgroup, cpu) pair that has
>>>>>>> updates, vmscan_flush aggregates cpu readings and propagates updates
>>>>>>> to parents.
>>>>>>>
>>>>>>> Detailed explanation:
>>>>>>> - The test loads tracing bpf programs, vmscan_start and vmscan_end, to
>>>>>>> measure the latency of cgroup reclaim. Per-cgroup ratings are stored in
>>>>>>> percpu maps for efficiency. When a cgroup reading is updated on a cpu,
>>>>>>> cgroup_rstat_updated(cgroup, cpu) is called to add the cgroup to the
>>>>>>> rstat updated tree on that cpu.
>>>>>>>
>>>>>>> - A cgroup_iter program, vmscan_dump, is loaded and pinned to a file, for
>>>>>>> each cgroup. Reading this file invokes the program, which calls
>>>>>>> cgroup_rstat_flush(cgroup) to ask rstat to propagate the updates for all
>>>>>>> cpus and cgroups that have updates in this cgroup's subtree. Afterwards,
>>>>>>> the stats are exposed to the user. vmscan_dump returns 1 to terminate
>>>>>>> iteration early, so that we only expose stats for one cgroup per read.
>>>>>>>
>>>>>>> - An ftrace program, vmscan_flush, is also loaded and attached to
>>>>>>> bpf_rstat_flush. When rstat flushing is ongoing, vmscan_flush is invoked
>>>>>>> once for each (cgroup, cpu) pair that has updates. cgroups are popped
>>>>>>> from the rstat tree in a bottom-up fashion, so calls will always be
>>>>>>> made for cgroups that have updates before their parents. The program
>>>>>>> aggregates percpu readings to a total per-cgroup reading, and also
>>>>>>> propagates them to the parent cgroup. After rstat flushing is over, all
>>>>>>> cgroups will have correct updated hierarchical readings (including all
>>>>>>> cpus and all their descendants).
>>>>>>>
>>>>>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>>>>>>
>>>>>> There are a selftest failure with test:
>>>>>>
>>>>>> get_cgroup_vmscan_delay:PASS:output format 0 nsec
>>>>>> get_cgroup_vmscan_delay:PASS:cgroup_id 0 nsec
>>>>>> get_cgroup_vmscan_delay:PASS:vmscan_reading 0 nsec
>>>>>> get_cgroup_vmscan_delay:PASS:read cgroup_iter 0 nsec
>>>>>> get_cgroup_vmscan_delay:PASS:output format 0 nsec
>>>>>> get_cgroup_vmscan_delay:PASS:cgroup_id 0 nsec
>>>>>> get_cgroup_vmscan_delay:FAIL:vmscan_reading unexpected vmscan_reading:
>>>>>> actual 0 <= expected 0
>>>>>> check_vmscan_stats:FAIL:child1_vmscan unexpected child1_vmscan: actual
>>>>>> 781874 != expected 382092
>>>>>> check_vmscan_stats:FAIL:child2_vmscan unexpected child2_vmscan: actual
>>>>>> -1 != expected -2
>>>>>> check_vmscan_stats:FAIL:test_vmscan unexpected test_vmscan: actual
>>>>>> 781874 != expected 781873
>>>>>> check_vmscan_stats:FAIL:root_vmscan unexpected root_vmscan: actual 0 <
>>>>>> expected 781874
>>>>>> destroy_progs:PASS:remove cgroup_iter pin 0 nsec
>>>>>> destroy_progs:PASS:remove cgroup_iter pin 0 nsec
>>>>>> destroy_progs:PASS:remove cgroup_iter pin 0 nsec
>>>>>> destroy_progs:PASS:remove cgroup_iter pin 0 nsec
>>>>>> destroy_progs:PASS:remove cgroup_iter pin 0 nsec
>>>>>> destroy_progs:PASS:remove cgroup_iter pin 0 nsec
>>>>>> destroy_progs:PASS:remove cgroup_iter pin 0 nsec
>>>>>> destroy_progs:PASS:remove cgroup_iter root pin 0 nsec
>>>>>> cleanup_bpffs:PASS:rmdir /sys/fs/bpf/vmscan/ 0 nsec
>>>>>> #33 cgroup_hierarchical_stats:FAIL
>>>>>>
>>>>>
>>>>> The test is passing on my setup. I am trying to figure out if there is
>>>>> something outside the setup done by the test that can cause the test
>>>>> to fail.
>>>>>
>>>>
>>>> I can't reproduce the failure on my machine. It seems like for some
>>>> reason reclaim is not invoked in one of the test cgroups which results
>>>> in the expected stats not being there. I have a few suspicions as to
>>>> what might cause this but I am not sure.
>>>>
>>>> If you have the capacity, do you mind re-running the test with the
>>>> attached diff1.patch? (and maybe diff2.patch if that fails, this will
>>>> cause OOMs in the test cgroup, you might see some process killed
>>>> warnings).
>>>> Thanks!
>>>>
>>>
>>> In addition to that, it looks like one of the cgroups has a "0" stat
>>> which shouldn't happen unless one of the map update/lookup operations
>>> failed, which should log something using bpf_printk. I need to
>>> reproduce the test failure to investigate this properly. Did you
>>> observe this failure on your machine or in CI? Any instructions on how
>>> to reproduce or system setup?
>>
>> I got "0" as well.
>>
>> get_cgroup_vmscan_delay:FAIL:vmscan_reading unexpected vmscan_reading:
>> actual 0 <= expected 0
>> check_vmscan_stats:FAIL:child1_vmscan unexpected child1_vmscan: actual
>> 676612 != expected 339142
>> check_vmscan_stats:FAIL:child2_vmscan unexpected child2_vmscan: actual
>> -1 != expected -2
>> check_vmscan_stats:FAIL:test_vmscan unexpected test_vmscan: actual
>> 676612 != expected 676611
>> check_vmscan_stats:FAIL:root_vmscan unexpected root_vmscan: actual 0 <
>> expected 676612
>>
>> I don't have special config. I am running on qemu vm, similar to
>> ci environment but may have a slightly different config.
>>
>> The CI for this patch set won't work since the sleepable kfunc support
>> patch is not available. Once you have that patch, bpf CI should be able
>> to compile the patch set and run the tests.
>>
>
> I will include this patch in the next version anyway, but I am trying
> to find out why this selftest is failing for you before I send it out.
> I am trying to reproduce the problem but no luck so far.

I debugged this a little bit and found that this two programs

SEC("tp_btf/mm_vmscan_memcg_reclaim_begin")
int BPF_PROG(vmscan_start, struct lruvec *lruvec, struct scan_control *sc)

and

SEC("tp_btf/mm_vmscan_memcg_reclaim_end")
int BPF_PROG(vmscan_end, struct lruvec *lruvec, struct scan_control *sc)

are not triggered.

I do have CONFIG_MEMCG enabled in my config file:
...
CONFIG_MEMCG=y
CONFIG_MEMCG_SWAP=y
CONFIG_MEMCG_KMEM=y
...

Maybe when cgroup_rstat_flush() is called, some code path won't trigger
mm_vmscan_memcg_reclaim_begin/end()?

>
>>>
>>>>
>>>>>>
>>>>>> Also an existing test also failed.
>>>>>>
>>>>>> btf_dump_data:PASS:find type id 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:PASS:failed/unexpected type_sz 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:FAIL:ensure expected/actual match unexpected ensure
>>>>>> expected/actual match: actual '(union bpf_iter_link_info){.map =
>>>>>> (struct){.map_fd = (__u32)1,},.cgroup '
>>>>>> test_btf_dump_struct_data:PASS:find struct sk_buff 0 nsec
>>>>>>
>>>>>
>>>>> Yeah I see what happened there. bpf_iter_link_info was changed by the
>>>>> patch that introduced cgroup_iter, and this specific union is used by
>>>>> the test to test the "union with nested struct" btf dumping. I will
>>>>> add a patch in the next version that updates the btf_dump_data test
>>>>> accordingly. Thanks.
>>>>>
>>>>>>
>>>>>> test_btf_dump_struct_data:PASS:unexpected return value dumping sk_buff 0
>>>>>> nsec
>>>>>>
>>>>>> btf_dump_data:PASS:verify prefix match 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:PASS:find type id 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:PASS:failed to return -E2BIG 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:PASS:ensure expected/actual match 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:PASS:verify prefix match 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:PASS:find type id 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:PASS:failed to return -E2BIG 0 nsec
>>>>>>
>>>>>>
>>>>>> btf_dump_data:PASS:ensure expected/actual match 0 nsec
>>>>>>
>>>>>>
>>>>>> #21/14 btf_dump/btf_dump: struct_data:FAIL
>>>>>>
>>>>>> please take a look.
>>>>>>
>>>>>>> ---
>>>>>>> .../prog_tests/cgroup_hierarchical_stats.c | 351 ++++++++++++++++++
>>>>>>> .../bpf/progs/cgroup_hierarchical_stats.c | 234 ++++++++++++
>>>>>>> 2 files changed, 585 insertions(+)
>>>>>>> create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
>>>>>>> create mode 100644 tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
>>>>>>>
>> …