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>
---
 .../prog_tests/cgroup_hierarchical_stats.c    | 364 ++++++++++++++++++
 .../bpf/progs/cgroup_hierarchical_stats.c     | 239 ++++++++++++
 2 files changed, 603 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

diff --git a/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c b/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
new file mode 100644
index 000000000000..1eafd94af4fe
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c

@@ -0,0 +1,364 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Functions to manage eBPF programs attached to cgroup subsystems
+ *
+ * Copyright 2022 Google LLC.
+ */
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include <test_progs.h>
+#include <bpf/libbpf.h>
+#include <bpf/bpf.h>
+
+#include "cgroup_helpers.h"
+#include "cgroup_hierarchical_stats.skel.h"
+
+#define PAGE_SIZE 4096
+#define MB(x) (x << 20)
+
+#define BPFFS_ROOT "/sys/fs/bpf/"
+#define BPFFS_VMSCAN BPFFS_ROOT"vmscan/"
+
+#define CG_ROOT_NAME "root"
+#define CG_ROOT_ID 1
+
+#define CGROUP_PATH(p, n) {.path = #p"/"#n, .name = #n}
+
+static struct {
+       const char *path, *name;
+       unsigned long long id;
+       int fd;
+} cgroups[] = {
+       CGROUP_PATH(/, test),
+       CGROUP_PATH(/test, child1),
+       CGROUP_PATH(/test, child2),
+       CGROUP_PATH(/test/child1, child1_1),
+       CGROUP_PATH(/test/child1, child1_2),
+       CGROUP_PATH(/test/child2, child2_1),
+       CGROUP_PATH(/test/child2, child2_2),

+{
+       char path[128];
+       int i;
+
+       for (i = 0; i < N_CGROUPS; i++) {
+               /* Delete files in bpffs that cgroup_iters are pinned in */
+               snprintf(path, 128, "%s%s", BPFFS_VMSCAN,
+                        cgroups[i].name);
+               ASSERT_OK(remove(path), "remove cgroup_iter pin");
+       }
+
+       /* Delete root file in bpffs */
+       snprintf(path, 128, "%s%s", BPFFS_VMSCAN, CG_ROOT_NAME);
+       ASSERT_OK(remove(path), "remove cgroup_iter root pin");
+       cgroup_hierarchical_stats__destroy(skel);
+}
+
+void test_cgroup_hierarchical_stats(void)
+{
+       struct cgroup_hierarchical_stats *skel = NULL;
+
+       if (setup_hierarchy())
+               goto hierarchy_cleanup;
+       if (setup_progs(&skel))
+               goto cleanup;
+       if (induce_vmscan())
+               goto cleanup;
+       check_vmscan_stats();
+cleanup:
+       destroy_progs(skel);
+hierarchy_cleanup:
+       destroy_hierarchy();
+}

diff --git a/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c b/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c
new file mode 100644
index 000000000000..85a65a72482e
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/cgroup_hierarchical_stats.c

@@ -0,0 +1,239 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Functions to manage eBPF programs attached to cgroup subsystems
+ *
+ * Copyright 2022 Google LLC.
+ */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+/*
+ * Start times are stored per-task, not per-cgroup, as multiple tasks in one
+ * cgroup can perform reclain concurrently.