Measuring semaphore contention times with markers

[Frank Ch. Eigler]

Mike Mason <mmlnx@us.ibm.com> writes:

> FWIW... Here's a simple example of using markers to measure
> semaphore contention times.  I don't claim that it's complete or
> efficient, just an example of an area where markers are useful.

Indeed.  (The equivalent systemtap script should be more compact.)

In order for upstream to consider including these sorts of valuable
markers, they'll want to have some benchmarks.  Most important is
bound to be the overall slowdown caused by dormant markers, and second
would be the increase in code (.text) size.  Of lesser interest might
be the slowdown caused by a minimal marker handler that just returns.
Can you or someone else run some tests?

> This was originally written by a colleague to use kprobes.  It
> probes in the middle of functions in some cases.  The kprobes
> version hard coded those probe point addresses, requiring the
> addresses to be updated for each kernel.  [...]

How hard did you try to use symbolic kernel.statement("*@file:line)
probes?  Would some of the outlined improvements (.relative and such)
in bugzilla have helped?


- FChE