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From:   Hao Luo <haoluo@google.com>
Date:   Thu, 25 Aug 2022 11:52:45 -0700
Message-ID: <CA+khW7hKRk-=s=eOJAGqJ9bap17kZu8p+YU+Bx-BUg+WaU2q_A@mail.gmail.com>
Subject: Re: [RESEND PATCH bpf-next v9 5/5] selftests/bpf: add a selftest for
 cgroup hierarchical stats collection
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Yosry Ahmed <yosryahmed@google.com>,
        LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
        Network Development <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>
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On Thu, Aug 25, 2022 at 11:43 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Wed, Aug 24, 2022 at 7:41 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Wed, Aug 24, 2022 at 7:09 PM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > On Wed, Aug 24, 2022 at 4:31 PM Hao Luo <haoluo@google.com> wrote:
> > > > +
> > > > +       for (i = 0; i < N_CGROUPS; i++) {
> > > > +               fd = create_and_get_cgroup(cgroups[i].path);
> > > > +               if (!ASSERT_GE(fd, 0, "create_and_get_cgroup"))
> > > > +                       return fd;
> > > > +
> > > > +               cgroups[i].fd = fd;
> > > > +               cgroups[i].id = get_cgroup_id(cgroups[i].path);
> > > > +
> > > > +               /*
> > > > +                * Enable memcg controller for the entire hierarchy.
> > > > +                * Note that stats are collected for all cgroups in a hierarchy
> > > > +                * with memcg enabled anyway, but are only exposed for cgroups
> > > > +                * that have memcg enabled.
> > > > +                */
> > > > +               if (i < N_NON_LEAF_CGROUPS) {
> > > > +                       err = enable_controllers(cgroups[i].path, "memory");
> > > > +                       if (!ASSERT_OK(err, "enable_controllers"))
> > > > +                               return err;
> > > > +               }
> > > > +       }
> > >
> > > It passes BPF CI, but fails in my setup with:
> > >
> > > # ./test_progs -t cgroup_hier -vv
> > > bpf_testmod.ko is already unloaded.
> > > Loading bpf_testmod.ko...
> > > Successfully loaded bpf_testmod.ko.
> > > setup_bpffs:PASS:mount 0 nsec
> > > setup_cgroups:PASS:setup_cgroup_environment 0 nsec
> > > setup_cgroups:PASS:get_root_cgroup 0 nsec
> > > setup_cgroups:PASS:create_and_get_cgroup 0 nsec
> > > (cgroup_helpers.c:92: errno: No such file or directory) Enabling
> > > controller memory:
> > > /mnt/cgroup-test-work-dir6526//test/cgroup.subtree_control
> > > setup_cgroups:FAIL:enable_controllers unexpected error: 1 (errno 2)
> > > cleanup_bpffs:FAIL:rmdir /sys/fs/bpf/vmscan/ unexpected error: -1 (errno 2)
> > > #36      cgroup_hierarchical_stats:FAIL
> > > Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED
> > >
> > > How do I debug it?
> >
> > The failure with ENOENT happens when we try to write "+memory" to
> > /mnt/cgroup-test-work-dir6526//test/cgroup.subtree_control, not when
> > we try to open it. So the file is there. AFAICT, ENOENT can be
> > returned from this write if the memory controller is not enabled on
> > this cgroup.
> >
> > In setup_cgroup_environment(), we should be enabling all available
> > controllers on /mnt and /mnt/cgroup-test-work-dir6526 by this line:
> >
> >         if (__enable_controllers(CGROUP_MOUNT_PATH, NULL) ||
> >               __enable_controllers(cgroup_workdir, NULL))
> >                   return 1;
> >
> > The first thing that comes to mind is that maybe the memory controller
> > is not enabled on your setup at all? Can you check
> > /sys/fs/cgroup/cgroup.controllers (or wherever your global cgroup
> > mount is)?
>
> Indeed. I didn't have a memory controller in cgroup2.
> My system booted with cgroup v1 and it had cgroup1 memory
> controller enabled which prevented cgroup2 to enable it.
> Without Tejun's help I would have been able to figure this out.
>
> Anyway, pushed the set to bpf-next. Thanks everyone.

Really awesome! Thanks everyone for the code review and the helpful
comments! Yosry and I can now start playing this new tool in our
production kernel. We will monitor for bugs and continue making
further improvements.