linux-kernel - Re: [RFC patch 00/15] Tracer Timestamping

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Date:	Fri, 17 Oct 2008 09:59:13 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>
Subject: Re: [RFC patch 00/15] Tracer Timestamping

On Thu, 2008-10-16 at 19:27 -0400, Mathieu Desnoyers wrote:
> Hi,
> 
> Starting with the bottom of my LTTng patchset
> (git://git.kernel.org/pub/scm/linux/kernel/git/compudj/linux-2.6-lttng.git)
> I post as RFC the timestamping infrastructure I have been using for a while in
> the tracer. It integrates get_cycles() standardization following David Miller's
> comments I did more recently.
> 
> It also deals with 32 -> 64 bits timestamp counter extension with a RCU-style
> algorithm, which is especially useful on MIPS and SuperH architectures.

Have you looked at the existing 32->63 extention code in
include/linux/cnt32_to_63.h and considered unifying it?

> There is also a TSC synchronization test within this patchset to detect
> unsynchronized TSCs. 

We already have such code, no? Does this code replace that one or just
add a second test?

> See comments in this specific patch to figure out the
> difference between the current x86 tsc_sync.c and the one I propose in this
> patch.

Right so you don't unify, that's a missed opportunity, no?

> It also provides an architecture-agnostic fallback in case there is no
> timestamp counter available : basically, it's
> (jiffies << 13) | atomically_incremented_counter (if there are more than 8192
> events per jiffy, time will still be monotonic, but will increment faster than
> the actual system frequency).
> 
> Comments are welcome. Note that this is only the beginning of the patchset. I
> plan to submit the event ID allocation/portable event typing aimed at exporting
> the data to userspace and buffering mechanism as soon as I integrate a core
> version of the LTTV userspace tools to the kernel build tree. Other than that, I
> currently have a tracer which fulfills most of the requirements expressed
> earlier. I just fear that if I release only the kernel part without foolproof
> binary-to-ascii trace decoder within the kernel, people might be a bit reluctant
> to fetch a separate userspace package.

It might be good to drop all the ltt naming and pick more generic names,
esp. as ftrace could use a lot of this infrastructure as well.

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