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From:   Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date:   Thu, 25 Aug 2022 01:09:08 +0200
Message-ID: <CAP01T77oxbfQnHSX5irq0d=srArq=ZTf_VAMuw0QNhfcjJVdKQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v9 5/5] selftests/bpf: add a selftest for cgroup
 hierarchical stats collection
To:     Hao Luo <haoluo@google.com>
Cc:     linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        cgroups@vger.kernel.org, netdev@vger.kernel.org,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        KP Singh <kpsingh@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Jiri Olsa <jolsa@kernel.org>, Michal Koutny <mkoutny@suse.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        David Rientjes <rientjes@google.com>,
        Stanislav Fomichev <sdf@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Yosry Ahmed <yosryahmed@google.com>
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On Thu, 25 Aug 2022 at 01:07, Hao Luo <haoluo@google.com> wrote:
>
> On Tue, Aug 23, 2022 at 8:01 PM Hao Luo <haoluo@google.com> wrote:
> >
> > From: Yosry Ahmed <yosryahmed@google.com>
> >
> > 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>
> > ---
>
> 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@posteo.net