On Mon, Aug 1, 2022 at 10:54 AM Hao Luo [off-list ref] wrote:

From: Yosry Ahmed <redacted>

From: Yosry Ahmed <redacted>

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 <redacted>
Signed-off-by: Hao Luo <redacted>
---
 .../prog_tests/cgroup_hierarchical_stats.c    | 358 ++++++++++++++++++
 .../bpf/progs/cgroup_hierarchical_stats.c     | 218 +++++++++++
 2 files changed, 576 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

[...]

+extern void cgroup_rstat_updated(struct cgroup *cgrp, int cpu) __ksym;
+extern void cgroup_rstat_flush(struct cgroup *cgrp) __ksym;
+
+static struct cgroup *task_memcg(struct task_struct *task)
+{
+       return task->cgroups->subsys[memory_cgrp_id]->cgroup;

memory_cgrp_id is kernel-defined internal enum which actually can
change based on kernel configuration (i.e., which cgroup subsystems
are enabled or not), is that right?

In practice you wouldn't hard-code it, it's better to use
bpf_core_enum_value() to capture enum's value in CO-RE-relocatable
way.

So it might be a good idea to demonstrate that here.

+}
+
+static uint64_t cgroup_id(struct cgroup *cgrp)
+{
+       return cgrp->kn->id;
+}
+

[...]