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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Mon, 30 Jan 2023 11:52:03 -0800
Message-ID: <CAADnVQJ7KxEK92qOz0Ya4MrACHpxngSpG4W38xuGEgZmXEG-vQ@mail.gmail.com>
Subject: Re: [PATCH 3/9] selftests/bpf: use canonical ftrace path
To:     Ross Zwisler <zwisler@chromium.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Ross Zwisler <zwisler@google.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-trace-kernel@vger.kernel.org,
        Mykola Lysenko <mykolal@fb.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        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>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Shuah Khan <shuah@kernel.org>, bpf <bpf@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>
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On Mon, Jan 30, 2023 at 10:19 AM Ross Zwisler <zwisler@chromium.org> wrote:
>
> The canonical location for the tracefs filesystem is at /sys/kernel/tracing.
>
> But, from Documentation/trace/ftrace.rst:
>
>   Before 4.1, all ftrace tracing control files were within the debugfs
>   file system, which is typically located at /sys/kernel/debug/tracing.
>   For backward compatibility, when mounting the debugfs file system,
>   the tracefs file system will be automatically mounted at:
>
>   /sys/kernel/debug/tracing
>
> Many tests in the bpf selftest code still refer to this older debugfs
> path, so let's update them to avoid confusion.

I wish that was the case, but in reality there are still systems
out there where tracefs is only mounted in that old location.
For example in one my VMs:

$ cat /proc/mounts |grep tracefs
tracefs /sys/kernel/debug/tracing tracefs rw,relatime 0 0
$ ls /sys/kernel/tracing/
$ uname -r
6.2.0-rc5-01030-gc1a3daf7363b

So this change will break the tests. We cannot do it.