linux-kernel - Re: [PATCH] tracing: Have trace_marker writes be just half of TRACE_SEQ

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Date: Mon, 4 Mar 2024 20:42:39 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
 Masami Hiramatsu <mhiramat@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Sachin Sant <sachinp@...ux.ibm.com>
Subject: Re: [PATCH] tracing: Have trace_marker writes be just half of
 TRACE_SEQ_SIZE

On 2024-03-04 20:41, Steven Rostedt wrote:
> On Mon, 4 Mar 2024 20:35:16 -0500
> Steven Rostedt <rostedt@...dmis.org> 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