Re: [PATCH bpf-next v3 8/8] bpf: add a selftest for cgroup hierarchical stats collection
From: Yonghong Song
Date: Sun Jul 10 2022 - 20:51:55 EST
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@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.
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
+}
+
+static int setup_cgroup_iter(struct cgroup_hierarchical_stats *obj,
int cgroup_fd,
[...]