linux-kernel - Re: Unified tracing buffer

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Date:	Mon, 22 Sep 2008 15:45:44 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Martin Bligh <mbligh@...gle.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Steven Rostedt <rostedt@...dmis.org>, od@...ell.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	systemtap-ml <systemtap@...rces.redhat.com>
Subject: Re: Unified tracing buffer

Hi Martin,

Martin Bligh wrote:
> During kernel summit and Plumbers conference, Linus and others
> expressed a desire for a unified
> tracing buffer system for multiple tracing applications (eg ftrace,
> lttng, systemtap, blktrace, etc) to use.
> This provides several advantages, including the ability to interleave
> data from multiple sources,
> not having to learn 200 different tools, duplicated code/effort, etc.
> 
> Several of us got together last night and tried to cut this down to
> the simplest usable system
> we could agree on (and nobody got hurt!). This will form version 1.
> I've sketched out a few
> enhancements we know that we want, but have agreed to leave these
> until version 2.
> The answer to most questions about the below is "yes we know, we'll
> fix that in version 2"
> (or 3). Simplicity was the rule ...
>
> Sketch of design.  Enjoy flaming me. Code will follow shortly.
>
>
> STORAGE
> -------
>
> We will support multiple buffers for different tracing systems, with
> separate names, event id spaces.
> Event ids are 16 bit, dynamically allocated.
> A "one line of text" print function will be provided for each event,
> or use the default (probably hex printf)
> Will provide a "flight data recorder" mode, and a "spool to disk" mode.
>
> Circular buffer per cpu, protected by per-cpu spinlock_irq
> Word aligned records.
> Variable record length, header will start with length record.
> Timestamps in fixed timebase, monotonically increasing (across all CPUs)

I agree to integrate tracing buffer mechanism, but I don't think
your proposal is the simplest one.

To simplify, I think the layered buffering mechanism is desirable.
- The lowest layer just provides named circular buffers(both per-cpu and
  uni-buffer in system) and read/write scheme.
- Next layer provides user/kernel interface including controls.
- Top layer defines packet(and event) formatting utilities.
- Additionally, it would better provide some library routines(timestamp,
  event-id synchronize, and so on).

Since this unified buffer is used from different kind of tracers/loggers,
I don't think all of them(and future tracers) want to be tied down by
"event-id"+"parameter" format.
So, Sorry, I disagree about that the tracing buffer defines its *data format*,
it's just overkill for me.

Thank you,


-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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