Re: [PATCH bpf-next v9 5/5] selftests/bpf: add a selftest for cgroup hierarchical stats collection

[Hao Luo]

On Wed, Aug 24, 2022 at 4:09 PM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> On Thu, 25 Aug 2022 at 01:07, Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > On Tue, Aug 23, 2022 at 8:01 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> > >
> > > 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>
> > > ---
> >
> > I saw this test failed on CI on s390x [0], because of using kfunc, and
> > on s390x, "JIT does not support calling kernel function". Is there
> > anything I can do about it
> >
>
> You can add it to the deny list, like this patch:
> https://lore.kernel.org/bpf/20220824163906.1186832-1-deso@xxxxxxxxxx

Very cool! Thanks!