Re: [PATCH bpf-next v3 8/8] bpf: add a selftest for cgroup hierarchical stats collection

Yosry Ahmed
2022-07-19T17:03:40Z

On Tue, Jul 19, 2022 at 9:17 AM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 7/18/22 12:34 PM, Yosry Ahmed wrote:
> > On Mon, Jul 11, 2022 at 8:55 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >>
> >> On Sun, Jul 10, 2022 at 5:51 PM Yonghong Song <yhs@fb.com> wrote:
> >>>
> >>>
> >>>
> >>> On 7/10/22 5:26 PM, Yonghong Song wrote:
> >>>>
> >>>>
> >>>> On 7/8/22 5:04 PM, Yosry Ahmed wrote:
> >>>>> Add a selftest that tests the whole workflow for collecting,
> >>>>> aggregating (flushing), and displaying cgroup hierarchical stats.
> >>>>>
> >>>>> TL;DR:
> >>>>> - Userspace program creates a cgroup hierarchy and induces memcg reclaim
> >>>>> in parts of it.
> >>>>> - 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.
> >>>>> - Userspace program makes sure the stats are aggregated and read
> >>>>> correctly.
> >>>>>
> >>>>> Detailed explanation:
> >>>>> - The test loads tracing bpf programs, vmscan_start and vmscan_end, to
> >>>>> measure the latency of cgroup reclaim. Per-cgroup readings 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).
> >>>>>
> >>>>> - Finally, the test creates a cgroup hierarchy and induces memcg reclaim
> >>>>> in parts of it, and makes sure that the stats collection, aggregation,
> >>>>> and reading workflow works as expected.
> >>>>>
> >>>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> >>>>> ---
> >>>>> .../prog_tests/cgroup_hierarchical_stats.c | 362 ++++++++++++++++++
> >>>>> .../bpf/progs/cgroup_hierarchical_stats.c | 235 ++++++++++++
> >>>>> 2 files changed, 597 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
> >>>>>
> >>>> [...]
> >>>>> +
> >>>>> +static unsigned long long get_cgroup_vmscan_delay(unsigned long long
> >>>>> cgroup_id,
> >>>>> + const char *file_name)
> >>>>> +{
> >>>>> + char buf[128], path[128];
> >>>>> + unsigned long long vmscan = 0, id = 0;
> >>>>> + int err;
> >>>>> +
> >>>>> + /* For every cgroup, read the file generated by cgroup_iter */
> >>>>> + snprintf(path, 128, "%s%s", BPFFS_VMSCAN, file_name);
> >>>>> + err = read_from_file(path, buf, 128);
> >>>>> + if (!ASSERT_OK(err, "read cgroup_iter"))
> >>>>> + return 0;
> >>>>> +
> >>>>> + /* Check the output file formatting */
> >>>>> + ASSERT_EQ(sscanf(buf, "cg_id: %llu, total_vmscan_delay: %llu\n",
> >>>>> + &id, &vmscan), 2, "output format");
> >>>>> +
> >>>>> + /* Check that the cgroup_id is displayed correctly */
> >>>>> + ASSERT_EQ(id, cgroup_id, "cgroup_id");
> >>>>> + /* Check that the vmscan reading is non-zero */
> >>>>> + ASSERT_GT(vmscan, 0, "vmscan_reading");
> >>>>> + return vmscan;
> >>>>> +}
> >>>>> +
> >>>>> +static void check_vmscan_stats(void)
> >>>>> +{
> >>>>> + int i;
> >>>>> + unsigned long long vmscan_readings[N_CGROUPS], vmscan_root;
> >>>>> +
> >>>>> + for (i = 0; i < N_CGROUPS; i++)
> >>>>> + vmscan_readings[i] = get_cgroup_vmscan_delay(cgroups[i].id,
> >>>>> + cgroups[i].name);
> >>>>> +
> >>>>> + /* Read stats for root too */
> >>>>> + vmscan_root = get_cgroup_vmscan_delay(CG_ROOT_ID, CG_ROOT_NAME);
> >>>>> +
> >>>>> + /* Check that child1 == child1_1 + child1_2 */
> >>>>> + ASSERT_EQ(vmscan_readings[1], vmscan_readings[3] +
> >>>>> vmscan_readings[4],
> >>>>> + "child1_vmscan");
> >>>>> + /* Check that child2 == child2_1 + child2_2 */
> >>>>> + ASSERT_EQ(vmscan_readings[2], vmscan_readings[5] +
> >>>>> vmscan_readings[6],
> >>>>> + "child2_vmscan");
> >>>>> + /* Check that test == child1 + child2 */
> >>>>> + ASSERT_EQ(vmscan_readings[0], vmscan_readings[1] +
> >>>>> vmscan_readings[2],
> >>>>> + "test_vmscan");
> >>>>> + /* Check that root >= test */
> >>>>> + ASSERT_GE(vmscan_root, vmscan_readings[1], "root_vmscan");
> >>>>
> >>>> I still get a test failure with
> >>>>
> >>>> 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 0
> >>>> != expected -2
> >>>> check_vmscan_stats:FAIL:child2_vmscan unexpected child2_vmscan: actual 0
> >>>> != expected -2
> >>>> check_vmscan_stats:PASS:test_vmscan 0 nsec
> >>>> check_vmscan_stats:PASS:root_vmscan 0 nsec
> >>>>
> >>>> I added 'dump_stack()' in function try_to_free_mem_cgroup_pages()
> >>>> and run this test (#33) and didn't get any stacktrace.
> >>>> But I do get stacktraces due to other operations like
> >>>> try_to_free_mem_cgroup_pages+0x1fd [kernel]
> >>>> try_to_free_mem_cgroup_pages+0x1fd [kernel]
> >>>> memory_reclaim_write+0x88 [kernel]
> >>>> cgroup_file_write+0x88 [kernel]
> >>>> kernfs_fop_write_iter+0xd0 [kernel]
> >>>> vfs_write+0x2c4 [kernel]
> >>>> __x64_sys_write+0x60 [kernel]
> >>>> do_syscall_64+0x2d [kernel]
> >>>> entry_SYSCALL_64_after_hwframe+0x44 [kernel]
> >>>>
> >>>> If you can show me the stacktrace about how
> >>>> try_to_free_mem_cgroup_pages() is triggered in your setup, I can
> >>>> help debug this problem in my environment.
> >>>
> >>> BTW, CI also reported the test failure.
> >>> https://github.com/kernel-patches/bpf/pull/3284
> >>>
> >>> For example, with gcc built kernel,
> >>> https://github.com/kernel-patches/bpf/runs/7272407890?check_suite_focus=true
> >>>
> >>> The error:
> >>>
> >>> get_cgroup_vmscan_delay:PASS:cgroup_id 0 nsec
> >>> get_cgroup_vmscan_delay:PASS:vmscan_reading 0 nsec
> >>> check_vmscan_stats:FAIL:child1_vmscan unexpected child1_vmscan:
> >>> actual 28390910 != expected 28390909
> >>> check_vmscan_stats:FAIL:child2_vmscan unexpected child2_vmscan:
> >>> actual 0 != expected -2
> >>> check_vmscan_stats:PASS:test_vmscan 0 nsec
> >>> check_vmscan_stats:PASS:root_vmscan 0 nsec
> >>>
> >>
> >> Hey Yonghong,
> >>
> >> Thanks for helping us debug this failure. I can reproduce the CI
> >> failure in my enviornment, but this failure is actually different from
> >> the failure in your environment. In your environment it looks like no
> >> stats are gathered for all cgroups (either no reclaim happening or bpf
> >> progs not being run). In the CI and in my environment, only one cgroup
> >> observes this behavior.
> >>
> >> The thing is, I was able to reproduce the problem only when I ran all
> >> test_progs. When I run the selftest alone (test_progs -t
> >> cgroup_hierarchical_stats), it consistently passes, which is
> >> interesting.
> >
> > I think I figured this one out (the CI failure). I set max_entries for
> > the maps in the test to 10, because I have 1 entry per-cgroup, and I
> > have less than 10 cgroups. When I run the test with other tests I
> > *think* there are other cgroups that are being created, so the number
> > exceeds 10, and some of the entries for the test cgroups cannot be
> > created. I saw a lot of "failed to create entry for cgroup.." message
> > in the bpf trace produced by my test, and the error turned out to be
> > -E2BIG. I increased max_entries to 100 and it seems to be consistently
> > passing when run with all the other tests, using both test_progs and
> > test_progs-no_alu32.
> >
> > Please find a diff attached fixing this problem and a few other nits:
> > - Return meaningful exit codes from the reclaimer() child process and
> > check them in induce_vmscan().
> > - Make buf and path variables static in get_cgroup_vmscan_delay()
> > - Print error code in bpf trace when we fail to create a bpf map entry.
> > - Print 0 instead of -1 when we can't find a map entry, to avoid
> > underflowing the unsigned counters in the test.
> >
> > Let me know if this diff works or not, and if I need to send a new
> > version with the diff or not. Also let me know if this fixes the
> > failures that you have been seeing locally (which looked different
> > from the CI failures).
>
> I tried this patch and the test passed in my local environment
> so the diff sounds good to me.
>

Awesome! Thanks so much for helping debugging this!

I will bundle this diff with Hao's cgroup_iter changes and send a v4 soon.

> >
> > Thanks!
> >
> >>
> >> Anyway, one failure at a time :) I am working on debugging the CI
> >> failure (that occurs only when all tests are run), then we'll see if
> >> fixing that fixes the problem in our environment as well.
> >>
> >> If you have any pointers about why a test would consistently pass
> >> alone and consistently fail with others that would be good. Otherwise,
> >> I will keep you updated with any findings I reach.
> >>
> >> Thanks again!
> >>
> >>>>
> >>>>> +}
> >>>>> +
> >>>>> +static int setup_cgroup_iter(struct cgroup_hierarchical_stats *obj,
> >>>>> int cgroup_fd,
> >>>> …