linux-kernel - Re: [PATCH 0/4] Linux Kernel Markers

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Message-ID: <OFAB3D8A6C.1643F2D3-ON80257262.000581E4-80257262.00088F04@uk.ibm.com>
Date:	Sat, 13 Jan 2007 01:33:29 +0000
From:	Richard J Moore <richardj_moore@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Andrew Morton <akpm@...l.org>, Greg Kroah-Hartman <gregkh@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, ltt-dev@...fik.org,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Ingo Molnar <mingo@...hat.com>,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	systemtap@...rces.redhat.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/4] Linux Kernel Markers

Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote on 20/12/2006
23:52:16:

> Hi,
>
> You will find, in the following posts, the latest revision of the Linux
Kernel
> Markers. Due to the need some tracing projects (LTTng, SystemTAP) has of
this
> kind of mechanism, it could be nice to consider it for mainstream
inclusion.
>
> The following patches apply on 2.6.20-rc1-git7.
>
> Signed-off-by : Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>

Mathiue, FWIW I like this idea. A few years ago I implemented something
similar, but that had no explicit clients. Consequently I made my hooks
code more generalized than is needed in practice. I do remember that Karim
reworked the LTT instrumentation to use hooks and it worked fine.

You've got the same optimizations for x86 by modifying an instruction's
immediate operand and thus avoiding a d-cache hit. The only real caveat is
the need to avoid the unsynchronised cross modification erratum. Which
means that all processors will need to issue a serializing operation before
executing a Marker whose state is changed. How is that handled?

One additional thing we did, which might be useful at some future point,
was adding a /proc interface. We reflected the current instrumentation
though /proc and gave the status of each hook. We even talked about being
able to enable or disabled instrumentation by writing to /proc but I don't
think we ever implemented this.

It's high time we settled the issue of instrumentation. It gets my vote,

Good luck!

Richard

- -
Richard J Moore
IBM Linux Technology Centre

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