Re: [PATCH bpf-next v2 8/8] bpf: add a selftest for cgroup hierarchical stats collection

From: Hao Luo <haoluo@google.com>
To: Yonghong Song <yhs@fb.com>
Cc: Yosry Ahmed <yosryahmed@google.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Tejun Heo <tj@kernel.org>,
	Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Shuah Khan <shuah@kernel.org>, Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	David Rientjes <rientjes@google.com>,
	Stanislav Fomichev <sdf@google.com>,
	Greg Thelen <gthelen@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Cgroups <cgroups@vger.kernel.org>
Subject: Re: [PATCH bpf-next v2 8/8] bpf: add a selftest for cgroup hierarchical stats collection
Date: Fri, 1 Jul 2022 16:28:11 -0700	[thread overview]
Message-ID: <CA+khW7jLLwgLBxpyn5s6Nc8V=0fjfH22193kjP1STbU2A+3v6Q@mail.gmail.com> (raw)
In-Reply-To: <59376285-21bc-ff12-3d64-3ea7257becb2@fb.com>

On Tue, Jun 28, 2022 at 11:27 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 6/28/22 12:43 AM, Yosry Ahmed wrote:
> > On Mon, Jun 27, 2022 at 11:47 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >>
> >> On Mon, Jun 27, 2022 at 11:14 PM Yonghong Song <yhs@fb.com> wrote:
[...]
> >>> btf_dump_data:FAIL:ensure expected/actual match unexpected ensure
> >>> expected/actual match: actual '(union bpf_iter_link_info){.map =
> >>> (struct){.map_fd = (__u32)1,},.cgroup '
> >>> test_btf_dump_struct_data:PASS:find struct sk_buff 0 nsec
> >>>
> >>
> >> Yeah I see what happened there. bpf_iter_link_info was changed by the
> >> patch that introduced cgroup_iter, and this specific union is used by
> >> the test to test the "union with nested struct" btf dumping. I will
> >> add a patch in the next version that updates the btf_dump_data test
> >> accordingly. Thanks.
> >>
> >
> > So I actually tried the attached diff to updated the expected dump of
> > bpf_iter_link_info in this test, but the test still failed:
> >
> > btf_dump_data:FAIL:ensure expected/actual match unexpected ensure
> > expected/actual match: actual '(union bpf_iter_link_info){.map =
> > (struct){.map_fd = (__u32)1,},.cgroup = (struct){.cgroup_fd =
> > (__u32)1,},}'  != expected '(union bpf_iter_link_info){.map =
> > (struct){.map_fd = (__u32)1,},.cgroup = (struct){.cgroup_fd =
> > (__u32)1,.traversal_order = (__u32)1},}'
> >
> > It seems to me that the actual output in this case is not right, it is
> > missing traversal_order. Did we accidentally find a bug in btf dumping
> > of unions with nested structs, or am I missing something here?
>
> Probably there is an issue in btf_dump_data() function in
> tools/lib/bpf/btf_dump.c. Could you take a look at it?
>

Regarding this failure of btf_dump_data, the cause seems that:

I added a new struct in 'union bpf_iter_link_info' in this patch
series, which expanded bpf_iter_link_info's size from 32bit to 64bit.
However, the test still used the old struct to initialize, which makes
a temporary stack variable (of type bpf_iter_link_info) partially
initialized. If I initialize the type by the larger new struct only,
btf_dump_data will output the correct content and the said test will
pass.

Yosry, we need to fold the said solution in the patch which introduced
changes to bpf_iter_link_info, so that it won't break the test.

I haven't dug into btf_dump_data() on why partially initialized union
fails. I need to look at the get_cgroup_vmscan_delay selftest in this
patch now.

Hao