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

Yosry Ahmed
2022-06-29T08:03:43Z

On Tue, Jun 28, 2022 at 11:27 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> 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?

I will try to take a look but after I figure out why the selftest
added here is always passing for me and always failing for you :(

>
> > 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
> >>>>
> …