Why trace the kernel?

Currently, LTTng implements support for user-space markers on x86 32 and 64 bits. It's a bit
slow, since it goes through a system call each time an event record must be recorded, and the
API is subject to change, but one can currently add markers to their userspace program or
library. See the package : http://ltt.polymtl.ca/packages/markers-userspace-0.5.tar.bz2. It
depends on the LTTng patchset to enable/disable markers and to record data. The tarball
contains examples telling how to modify the makefiles and linker scripts to use markers in
userspace.

Being the LTTng project lead, I dream about a simple in-kernel API to manage the performance
counters, which would aim at managing these limited resources for the various users (watchdog,
user-space perfmon-like API, in-kernel LTTng). The tracer is itself easily extensible and can
record new events which include performance counters either in an interrupt mode or at
specific events occuring on the system (system call entry/exit, interrupt handler entry/exit,
trap entry/exit...). I just need something to setup these counters and let them run free on
the system without changing them when switching from one task to another : this is something
really annoying when gathering system-wide information. I haven't looked at the perfmon code
lately, but I think that most of the user-space system call API is useless to an in-kernel
user like LTTng. A minimalistic perfmon would be welcome.