selftests/bpf: Simplify cgroup_hierarchical_stats selftest

The cgroup_hierarchical_stats selftest is complicated. It has to be,
because it tests an entire workflow of recording, aggregating, and
dum

selftests/bpf: Simplify cgroup_hierarchical_stats selftest

The cgroup_hierarchical_stats selftest is complicated. It has to be,
because it tests an entire workflow of recording, aggregating, and
dumping cgroup stats. However, some of the complexity is unnecessary.
The test now enables the memory controller in a cgroup hierarchy, invokes
reclaim, measure reclaim time, THEN uses that reclaim time to test the
stats collection and aggregation. We don't need to use such a
complicated stat, as the context in which the stat is collected is
orthogonal.

Simplify the test by using a simple stat instead of reclaim time, the
total number of times a process has ever entered a cgroup. This makes
the test simpler and removes the dependency on the memory controller and
the memory reclaim interface.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: KP Singh <kpsingh@kernel.org>
Link: https://lore.kernel.org/bpf/20220919175330.890793-1-yosryahmed@google.com

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selftests/bpf: add a selftest for cgroup hierarchical stats collection

Add a selftest that tests the whole workflow for collecting,
aggregating (flushing), and displaying cgroup hierarchical stats.

selftests/bpf: add a selftest for cgroup hierarchical stats collection

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>
Signed-off-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/r/20220824233117.1312810-6-haoluo@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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