linux-kernel - Re: [PATCH 4/5] ftrace: show ftrace

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Date:	Wed, 14 Jan 2009 09:25:46 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] ftrace: show ftrace_bprintk()'s formats

On Wed, Jan 14, 2009 at 10:49:56AM +0800, Lai Jiangshan wrote:
> Jason Baron wrote:
> > On Wed, Dec 31, 2008 at 10:56:23AM +0800, Lai Jiangshan wrote:
> >> Impact: let user knows the format
> >>
> >> Create a file on <debugfs-dir>/tracing/ to show ftrace_bprintk()'s formats.
> >>
> >> This formats will help for these condition:
> >> 1) User get binary data from core file.(formats are backup before coredump)
> >> 2) User splice ring_buffer to a file.
> >> User can use formats for parsing the binary data in userspace.
> >>
> > 
> > When I 'cat' trace_bprintk_formats on my system the file is empty. This
> > seems to be b/c 'ftrace_bprintk' is not being used in this patchset. It
> > can't be used in patch #5 during marker register b/c the format wouldn't
> > be known at runtime. Thus, as it currently stands this patch, patch 4/5,
> > isn't adding much?
> 
> If you don't use ftrace_bprintk(), the file trace_bprintk_formats is empty.
> This file record all formats which ftrace_bprintk() uses.
> 
> You can use ftrace_bprintk() everywhere as another printk().
> 
> Patch #5 enables binary printk for markers, this is another additional tool
> for tracing markers.
> 
> > 
> > A thought on how this might be resolved would be to have the core marker
> > code pass us its address so this could be recorded in the trace buffer.
> > Then, also add some debug file that displays the markers and maps marker
> > addresses with format strings.
> > 
> 
> Patch#5 does it as you said.
> 
> 
> 

hmm...i'm still not clear on this. in patch #5 you have:


+static void marker_bprintk_probe(void *probe_private, void *call_private,
+		const char *fmt, va_list *args)
+{
+	struct marker_bprintk_struct *p = probe_private;
+
+	if (p->fmt_state == MARKER_FMT_LACK) {
+		int flen = strlen(fmt);
+		if (p->fmt - p->name + flen < MARKER_NAME_FMT_LEN) {
+			memcpy(p->fmt, fmt, flen + 1);
+			p->fmt_state = MARKER_FMT_OK;
+		} else
+			p->fmt_state = MARKER_FMT_INVALID;
+	}
+
+	if (p->fmt_state == MARKER_FMT_OK)
+		trace_vbprintk(0, p->name, *args);
+}
+

trace_vbprintk first argument is 'ip'. So I don't see how we are
associating instruction pointers with each 'ftrace' record? Aren't we
just recording each entry with '0' for the 'ip'?

thanks,

-Jason


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