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

Yosry Ahmed
2022-06-29T08:04:43Z

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.

> >
> >>
> >>>>
> >>>> 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
> >>>>>
> …