On 6/28/22 12:43 AM, Yosry Ahmed wrote:

On Mon, Jun 27, 2022 at 11:47 PM Yosry Ahmed [off-list ref] wrote:

quoted

On Mon, Jun 27, 2022 at 11:14 PM Yonghong Song [off-list ref] wrote:

quoted

On 6/10/22 12:44 PM, Yosry Ahmed wrote:

quoted

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 <redacted>

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.

quoted

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!

quoted

quoted

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.

quoted

---
   .../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

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