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

[Yonghong Song]

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 ..."