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From:   Hao Luo <haoluo@google.com>
Date:   Mon, 29 Aug 2022 18:42:08 -0700
Message-ID: <CA+khW7g-jeiXMM-K+KK7L3tzG0catFSM+x5vHKMs=PF=s+=Pag@mail.gmail.com>
Subject: Re: [PATCH] selftests/bpf: simplify cgroup_hierarchical_stats selftest
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Andrii Nakryiko <andrii@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Mykola Lysenko <mykolal@fb.com>, Song Liu <song@kernel.org>,
        Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Jiri Olsa <jolsa@kernel.org>, bpf <bpf@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Aug 29, 2022 at 6:07 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Mon, Aug 29, 2022 at 3:15 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Mon, Aug 29, 2022 at 1:08 PM Hao Luo <haoluo@google.com> wrote:
> > >
> > > On Fri, Aug 26, 2022 at 4:06 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> > > >
[...]
> > > >
> > > > -SEC("tp_btf/mm_vmscan_memcg_reclaim_begin")
> > > > -int BPF_PROG(vmscan_start, int order, gfp_t gfp_flags)
> > > > +SEC("fentry/cgroup_attach_task")
> > >
> > > Can we select an attachpoint that is more stable? It seems
> > > 'cgroup_attach_task' is an internal helper function in cgroup, and its
> > > signature can change. I'd prefer using those commonly used tracepoints
> > > and EXPORT'ed functions. IMHO their interfaces are more stable.
> > >
> >
> > Will try to find a more stable attach point. Thanks!
>
> Hey Hao,
>
> I couldn't find any suitable stable attach points under kernel/cgroup.
> Most tracepoints are created using TRACE_CGROUP_PATH which only
> invokes the tracepoint if the trace event is enabled, which I assume
> is not something we can rely on. Otherwise, there is only

Can we explicitly enable the cgroup_attach_task event, just for this
test? If it's not easy, I am fine with using fentry.

> trace_cgroup_setup_root() and trace_cgroup_destroy_root() which are
> irrelevant here. A lot of EXPORT'ed functions are not called in the
> kernel, or cannot be invoked from userspace (the test) in a
> straightforward way. Even if they did, future changes to such code
> paths can also change in the future, so I don't think there is really
> a way to guarantee that future changes don't break the test.
>
> Let me know what you think.
>