Re: [PATCH] selftests/bpf: simplify cgroup_hierarchical_stats selftest

[Yosry Ahmed]

On Fri, Sep 9, 2022 at 5:50 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Sep 6, 2022 at 2:35 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >
> > On Mon, Aug 29, 2022 at 6:50 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > >
> > > On Mon, Aug 29, 2022 at 6:42 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> > > >
> > > > On Mon, Aug 29, 2022 at 6:07 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Aug 29, 2022 at 3:15 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Mon, Aug 29, 2022 at 1:08 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Fri, Aug 26, 2022 at 4:06 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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.
> > >
> > > I see a couple of tests that read from /sys/kernel/debug/tracing, but
> > > they are mostly reading event ids, I don't see any tests enabling or
> > > disabling a tracing event, so I am not sure if that's an accepted
> > > pattern. Also I am not sure if we can rely on tracefs being in that
> > > path. Andrii, is this considered acceptable?
> > >
> >
> > Anyone with thoughts here? Is it acceptable to explicitly enable a
> > trace event in a BPF selftest to attach to a tracepoint that is only
> > invoked if the trace event is enabled (e.g. cgroup_attach_task) ?
> > Otherwise the test program would attach to the fentry of an internal
> > function, which is more vulnerable to being changed and breaking the
> > test (until someone updates the test with the new signature).
> >
>
> IMO it's fine to use fentry. If something changes about signature,
> we'll detect it soon enough and adjust selftests.
>
> Messing with global tracefs in selftests is less desirable. It will
> also potentially force tests to be sequential.
>

Undestood. Thanks Andrii.
Will send v2 with other comments from KP and Hao.

> > > >
> > > > > 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.
> > > > >