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

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Message-ID: <20240304204119.7503ab0b@gandalf.local.home>
Date: Mon, 4 Mar 2024 20:41:19 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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 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);

-- Steve