Re: [PATCH] tracing: Have trace_marker writes be just half of TRACE_SEQ_SIZE

[Mathieu Desnoyers]

On 2024-03-04 20:41, Steven Rostedt wrote:
> On Mon, 4 Mar 2024 20:35:16 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > > BUILD_BUG_ON(TRACING_MARK_MAX_SIZE + sizeof(meta data stuff...) > TRACE_SEQ_SIZE);
> >
> > That's not the meta size I'm worried about. The sizeof(meta data) is the
> > raw event binary data, which is not related to the size of the event output.
> >
> >   # cd /sys/kernel/tracing
> >   # echo hello > trace_marker
> >   # cat trace
> > [..]
> >             <...>-999     [001] .....  2296.140373: tracing_mark_write: hello
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >     This is the meta data that is added to trace_seq
>
> That said, the meta data is most likely not going to be more than 128 bytes
> (it shouldn't be more than 80).
>
> I could do as you suggest and create a separate TRACE_MARKER_SIZE and just
> make sure that it's less than TRACE_SEQ_BUFFER_SIZE (as that's the size of
> the content) by 128 bytes.
>
> /* Added meta data should not be more than 128 bytes */
> BUILD_BUG_ON((TRACE_MARKER_MAX_SIZE + 128) > TRACE_SEQ_BUFFER_SIZE);

Bonus points if you add

#define TRACE_OUTPUT_META_DATA_MAX_LEN		80

and a runtime check in the code generating this header.

This would avoid adding an unchecked upper limit.

Thanks,

Mathieu

> -- Steve

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com