Re: [PATCH bpf-next v5 8/8] bpf: add a selftest for cgroup hierarchical stats collection

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On 7/22/22 10:48 AM, 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>

Let us tag the subject with "selftests/bpf: Add a selftest ..." instead
of "bpf: add a selftest ..."



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