linux-kernel - Re: [ANNOUNCE] New utility: 'trace'

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Date:	Tue, 16 Nov 2010 23:17:28 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Jason Baron <jbaron@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Christoph Hellwig <hch@....de>,
	Pekka Enberg <penberg@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [ANNOUNCE] New utility: 'trace'

On Tue, Nov 16, 2010 at 01:27:35PM -0800, Darren Hart wrote:
> On 11/16/2010 01:04 PM, Thomas Gleixner wrote:
>> We are pleased to announce a new tracing tool called 'trace'.
>>
>
> Hi Thomas, Ingo - Congrats and Thanks!
>
>> At this point we'd like to ask for feedback from other users of tracing tools,
>> which workflow components would you like to see in addition to what the 'trace'
>> tool is offering?
>>
>> Comments, bug reports, suggestions welcome,
>
> The first thing that comes to mind is trace_marker:
>
> echo "Test point A" > /sys/kernel/debug/tracing/trace_marker
>
> I've found this sort of markup to be useful using ftrace (with C  
> equivalents embedded in the test case). Is this supported?


No, the trace_marker file sends trace_printk events. And trace_printk
events needs to be converted to use the unified trace event interface
(ie: implement a struct ftrace_event_call).

This shouldn't be so hard, and there are several ways we could do this.

An idea is to reproduce the kernel file hierarchy in a "printk" event
subsystem, but this implies to allow subsystems nesting.

Imagine you have two trace_printk(), one in kernel/sched.c:117
and one in kernel/time/tick-sched.c:228
The result would be:

$ ls /sys/kernel/debug/tracing/events/printk

/sys/kernel/debug/tracing/events/printk:
enable  filter  kernel/

/sys/kernel/debug/tracing/events/printk/kernel:
enable  filter sched.c/ time/

/sys/kernel/debug/tracing/events/printk/kernel/sched.c:
enable  filter  117/

/sys/kernel/debug/tracing/events/printk/kernel/sched.c/117:
enable filter format id

/sys/kernel/debug/tracing/events/printk/kernel/time:
enable  filter tick-sched.c/

/sys/kernel/debug/tracing/events/printk/kernel/time/tick-sched.c:
enable  filter  228/

/sys/kernel/debug/tracing/events/printk/kernel/time/tick-sched.c/228:
enable filter format id


The format would be quite easy, and only one field for the whole string (strloc).

Not sure what to do with trace_marker. I just think we can add it as:
/sys/kernel/debug/tracing/events/printk/sys/kernel/debug/tracing/trace_marker

<:o)


But may be the whole idea is just fancy and nobody will care, and would just
a simple single event in a printk subsystem, on which you can use a kind of
virtual filter:

	echo "path != kernel/whatever.c:226" > /sys/kernel/debug/tracing/events/printk/filter

would turn on every trace_printk() but the one in the given path.

Dunno, I like both ideas. I prefer the first one which looks to me more flexible: could be
useful to list the user his trace_printks for example and implement an interface for him
to quickly select the trace_printk he wants.

For example I'm currently working with dozens of trace_printk() and I would be very happy
to turn some of them off half of the time.

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