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

From: Yonghong Song
Date: Wed Jun 29 2022 - 02:49:00 EST



On 6/28/22 5:09 PM, Yosry Ahmed wrote:
On Tue, Jun 28, 2022 at 12:14 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:

On Mon, Jun 27, 2022 at 11:47 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:

On Mon, Jun 27, 2022 at 11:14 PM Yonghong Song <yhs@xxxxxx> 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@xxxxxxxxxx>

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.




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

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