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

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Message-ID: <20240304203516.45b7a551@gandalf.local.home>
Date: Mon, 4 Mar 2024 20:35:16 -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 20:15:57 -0500
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> On 2024-03-04 19:27, Steven Rostedt wrote:
> > From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> > 
> > Since the size of trace_seq's buffer is the max an event can output, have
> > the trace_marker be half of the entire TRACE_SEQ_SIZE, which is 4K. That
> > will keep writes that has meta data written from being dropped (but
> > reported), because the total output of the print event is greater than
> > what the trace_seq can hold.  
> 
> Defining the trace_mark limit in terms of "TRACE_SEQ_SIZE / 2"
> seems backwards. It's basically using a define of the maximum
> buffer size for the pretty-printing output and defining the maximum
> input size of a system call to half of that.
> 
> I'd rather see, in a header file shared between tracing mark
> write implementation and output implementation:
> 
> #define TRACING_MARK_MAX_SIZE	4096
> 
> and then a static validation that this input fits within your
> pretty printing output in the output implementation file:
> 
> 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

-- Steve



> 
> This way we clearly document that the tracing mark write
> input limit is 4kB, rather than something derived from
> the size of an output buffer.
> 
> Thanks,
> 
> Mathieu
> 
> > 
> > Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> > ---
> >   kernel/trace/trace.c | 16 +++++++++++-----
> >   1 file changed, 11 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 8198bfc54b58..d68544aef65f 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -7320,6 +7320,17 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
> >   	if ((ssize_t)cnt < 0)
> >   		return -EINVAL;
> >   
> > +	/*
> > +	 * TRACE_SEQ_SIZE is the total size of trace_seq buffer used
> > +	 * for output. As the print event outputs more than just
> > +	 * the string written, keep it smaller than the trace_seq
> > +	 * as it could drop the event if the extra data makes it bigger
> > +	 * than what the trace_seq can hold. Half he TRACE_SEQ_SIZE
> > +	 * is more than enough.
> > +	 */
> > +	if (cnt > TRACE_SEQ_SIZE / 2)
> > +		cnt = TRACE_SEQ_SIZE / 2;
> > +
> >   	meta_size = sizeof(*entry) + 2;  /* add '\0' and possible '\n' */
> >    again:
> >   	size = cnt + meta_size;
> > @@ -7328,11 +7339,6 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
> >   	if (cnt < FAULTED_SIZE)
> >   		size += FAULTED_SIZE - cnt;
> >   
> > -	if (size > TRACE_SEQ_BUFFER_SIZE) {
> > -		cnt -= size - TRACE_SEQ_BUFFER_SIZE;
> > -		goto again;
> > -	}
> > -
> >   	buffer = tr->array_buffer.buffer;
> >   	event = __trace_buffer_lock_reserve(buffer, TRACE_PRINT, size,
> >   					    tracing_gen_ctx());  
>