Re: Static instrumentation, was Re: RFC: klogger: kernel tracingand logging tool

From: Yoav Etsion
Date: Wed Feb 22 2006 - 15:58:12 EST


Frank,

You raise two important issues:
1. code markers/annotations for tracing/probing purposes.
2. overhead of the kernel loggers in their inactive state

Of these, I think the first is more important, as it addresses some basic defeciency of software development --- getting to know someone else's code.
In my experience, writing instrumentation for a kernel subsystem (schema in Klogger lingo) requires in depth understanding of the code. This sometimes tunnel tremendous efforts towards measurements that could otherwise become trivial.

Since no one knows the code like its coder, having developers annotate their code using some semi-formal language/definitions (or even compiler pragmas) can serve as the best basis for any kernel logger.
Once such markers are in place, the second issue --- overheads (as most anything else)--- becomes a technical issue. So even when incurring inactive overheads, such a tool can be very useful for developers and researchers alike.

After all my babble, the bottom line to the community:
will kernel developers annotate their code? can such policies be instated?

Yoav


Frank Ch. Eigler wrote:
Yoav Etsion <etsman@xxxxxxxxx> writes:


[...] I've developed a kernel logging tool called
Klogger: http://www.cs.huji.ac.il/~etsman/klogger
In some senses, it is similar to the LTT [...]


It seems like several projects would benefit from markers being
inserted into key kernel code paths for purposes of tracing / probing.

Both LTTng and klogger have macros that expand to largish inline
function calls that appear to cause a noticeable amount of work even
for tracing requests are not actually active. (klogger munges
interrupts, gets timestamps, before ever testing whether logging was
requested; lttng similar; icache bloat in both cases.)

In other words, even in the inactive state, tracing markers like those
of klogger and ltt impose some performance disruption. Assuming that
detailed tracing / probing would be a useful facility to have
available, are there any other factors that block adoption of such
markers in official kernels? In other words, if they could go "fast
enough", especially in the inactive case, would you start placing them
into your code?


- FChE

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