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Date:   Tue, 28 Jun 2022 23:26:51 -0700
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Subject: Re: [PATCH bpf-next v2 8/8] bpf: add a selftest for cgroup
 hierarchical stats collection
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To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shuah Khan <shuah@kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        David Rientjes <rientjes@google.com>,
        Stanislav Fomichev <sdf@google.com>,
        Greg Thelen <gthelen@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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References: <20220610194435.2268290-1-yosryahmed@google.com>
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On 6/28/22 12:43 AM, Yosry Ahmed 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.
>>
>>>
>>> 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.
>>
> 
> So I actually tried the attached diff to updated the expected dump of
> bpf_iter_link_info in this test, but the test still failed:
> 
> 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 = (struct){.cgroup_fd =
> (__u32)1,},}'  != expected '(union bpf_iter_link_info){.map =
> (struct){.map_fd = (__u32)1,},.cgroup = (struct){.cgroup_fd =
> (__u32)1,.traversal_order = (__u32)1},}'
> 
> It seems to me that the actual output in this case is not right, it is
> missing traversal_order. Did we accidentally find a bug in btf dumping
> of unions with nested structs, or am I missing something here?

Probably there is an issue in btf_dump_data() function in
tools/lib/bpf/btf_dump.c. Could you take a look at it?

> 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
>>>>
[...]