[Qemu-devel] DPRINTF vs tracing

[Stefan Hajnoczi]

Historically, QEMU code has defined per-file DPRINTF() macros for
debug logging.  A lot of recent patches continue to use this
technique.

I want to draw attention to tracing support and its advantages over
DPRINTF().  Tracing is an alternative that solves limitations of the
DPRINTF() approach:

1. DPRINTF() requires editing a specific source file and recompiling.

While fine for the original author during development, DPRINTF() are
not available to users hitting issues in production.  I have not seen
any bug report threads where users are urged to recompile with a
DPRINTF() enabled.  That's because it just isn't practical and we're
losing out on a great source of debug information.

With tracing it is now possible for distros to ship with SystemTap
support (although LTTng Userspace Tracer and a portable built-in
tracer are also supported).  I hope we'll soon be able to take
advantage of this to get detailed information from users experiencing
bugs.  Let's start adding useful trace events now so we can ask users
to report detailed information in the future.

2. DPRINTF() is file-wide and not selectively filtered.

DPRINTF() can only be enabled/disabled for an entire source file at
compile time.  This leads to noisy outputs that need to be
post-filtered for the specific DPRINTF() you were looking for.

Trace events can be enabled on a per-event basis.  They can be toggled
at run-time without recompiling.  Less noise and less recompiling
makes debugging easier.

3. DPRINTF() duplicates code.

People have recently pointed out that the number of copy-pasted
DPRINTF() definitions are ridiculous.

Tracing allows you to focus on defining and calling specific events
without the boilerplate.  Add one line to the trace-events file and
call it from your source file, that's it.  You don't need to know
LTTng UST or SystemTap/DTrace.

For more information on tracing:
http://git.qemu.org/qemu.git/plain/docs/tracing.txt

See all existing trace events today:
http://git.qemu.org/qemu.git/plain/trace-events

Stefan