Re: [PATCH bpf-next v6 8/8] selftests/bpf: add a selftest for cgroup hierarchical stats collection

[Andrii Nakryiko]

On Mon, Aug 1, 2022 at 10:54 AM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
>
> From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
>
> 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>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---
>  .../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;
> +}
> +

[...]