linux-kernel - Re: [RFC][PATCH 0/2] tracing: Have trace

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Date:	Thu, 18 Nov 2010 13:39:43 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"jason.wessel" <jason.wessel@...driver.com>,
	Ted Ts'o <tytso@....edu>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [RFC][PATCH 0/2] tracing: Have trace_printk()s in the events/
	directory

On Wed, Nov 17, 2010 at 10:58:03PM -0500, Steven Rostedt wrote:
> As suggested in:
> 
> LKML-Reference: <20101116221726.GB26243@...here>
>   or
> http://marc.info/?l=linux-kernel&m=128994587230629
> 
> To have trace_printks in the events directory.

(Note, I don't remember who got that idea. Perhaps
that was Ingo while discussing that with him like one year
ago, or may be me, not sure, whatever)...


> This patch set just does it for ftrace, although it can also lay the
> ground work for both perf and trace (are they different tools?)

The trace tool uses perf. But no worry about perf, I can handle the hook
for it.


> This patch set converts the current trace_printk() into ftrace_printk()
> (back to it's original name) and that stays the same.
> 
> The new trace_printk() is off by default (we can change that later
> with a kernel command line option perhaps), but it can be enable now
> with the events directory.

Cool. So disablement/re-enablement are nice features. That said it should
be enabled by default I think, to keep the previous (and intuitive) behaviour.

 
> Each trace_printk() added will get a entry in the debugfs/tracing/events/
> directory as  debugfs/tracing/entry/path/to/trace_printk/
> 
> For example, I added a trace_printk() in kernel/sched.c at line 2180
> and it creates:
> 
> # ls /debug/tracing/events/printk/kernel/sched.c/2180/
> enable  format

Ok, we'll probably need to add the ID as well.

 
> The format is the printk format:
> 
> # cat /debug/tracing/events/printk/kernel/sched.c/2180/format 
> "migrate task %s:%d
> "

Do you think we can make it more compatible with the common pattern?

Like:

Name: sched.c:2180
ID: 120
format:
	field:unsigned long ip;	offset:16;	size:8;	signed:0;
	field:const char * fmt;	offset:24;	size:8;	signed:0; <-- can be retrieved from printk_format
	field:u32 buf;	offset:32;	size:0;	signed:0;

print fmt: "given format", REC->ip, REC->fm


Does that look possible to you or have you run into troubles?

I don't know how much that's possible given the bprintk format. May be
we should first try with the already formatted trace_printk version.


> Every directory has an "enable" file. By echoing 0 or 1 into this
> file, it will enable all the trace_printk's below it.
> 
> This also uses jump_label() to keep overhead down when disabled.
> 
> Note, modules are not (yet) supported.
> 
> This is just the ground work. I'm busy doing other things so
> if you need more, feel free to take this and carry on.
> 
> -- Steve


Thanks a lot!


> 
> The following patches are in:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
> 
>     branch: rfc/event-printk
> 
> 
> Steven Rostedt (2):
>       tracing: Rename trace_printk to ftrace_printk
>       tracing: Make event based trace_printk()
> 
> ----
>  include/asm-generic/vmlinux.lds.h |    7 +-
>  include/linux/kernel.h            |   16 +-
>  include/trace/event_printk.h      |   52 ++++++
>  kernel/trace/Makefile             |    1 +
>  kernel/trace/event_printk.c       |  365 +++++++++++++++++++++++++++++++++++++
>  kernel/trace/trace.h              |    1 +
>  kernel/trace/trace_events.c       |    2 +-
>  7 files changed, 435 insertions(+), 9 deletions(-)

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