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

[Yonghong Song]

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@xxxxxxxxxx>
> ---
>   .../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.

> +}
> +
> +static int setup_cgroup_iter(struct cgroup_hierarchical_stats *obj, int cgroup_fd,
[...]