Re: [PATCH 0/4] Linux Kernel Markers

[Richard J Moore <richardj_moore@uk.ibm.com>]

Mathieu Desnoyers <mathieu.desnoyers@polymtl.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@polymtl.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