netdev - Re: [PATCH v2 3/4] bpf: clarify when bpf_trace

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Date:   Tue, 20 Aug 2019 16:22:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Wu <peter@...ensteyn.nl>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH v2 3/4] bpf: clarify when bpf_trace_printk discards lines

On Wed, Aug 21, 2019 at 12:08:59AM +0100, Peter Wu wrote:
> I opened /sys/kernel/tracing/trace once and kept reading from it.
> bpf_trace_printk somehow did not seem to work, no entries were appended
> to that trace file. It turns out that tracing is disabled when that file
> is open. Save the next person some time and document this.
> 
> The trace file is described in Documentation/trace/ftrace.rst, however
> the implication "tracing is disabled" did not immediate translate to
> "bpf_trace_printk silently discards entries".
> 
> Signed-off-by: Peter Wu <peter@...ensteyn.nl>
> ---
>  include/uapi/linux/bpf.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 9ca333c3ce91..e4236e357ed9 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -575,6 +575,8 @@ union bpf_attr {
>   * 		limited to five).
>   *
>   * 		Each time the helper is called, it appends a line to the trace.
> + * 		Lines are discarded while *\/sys/kernel/debug/tracing/trace* is
> + * 		open, use *\/sys/kernel/debug/tracing/trace_pipe* to avoid this.

that's not quite correct.
Having 'trace' file open doesn't discard lines.
I think this type of comment in uapi header makes more confusion than helps.