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 <pe...@lekensteyn.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.
