Re: [PATCH 0/5] trace: inroduce qmp: trace namespace
Vladimir Sementsov-Ogievskiy <vsementsov(a)virtuozzo.com> writes:
Hi all!
We have handle_qmp_command and qmp_command_repond trace points to trace
qmp commands. They are very useful to debug problems involving
management tools like libvirt.
But tracing all qmp commands is too much.
Here I suggest a kind of tracing namespace. Formally this series adds a
trace points called qmp:<some-command> for every command, which may be
enabled in separate like
--trace qmp:drive-backup
or by pattern like
--trace qmp:block-job-*
or similarly with help of qmp command trace-event-set-state.
This also allows to enable tracing of some qmp commands permanently
(by downstream patch or in libvirt xml). For example, I'm going to
enable tracing of block job comamnds and blockdev-* commands in
Virtuozzo. Qemu logs are often too empty (for example, in comparison
with Libvirt), logging block jobs is not too much but will be very
helpful.
What exactly is traced? Peeking at PATCH 5... looks like it's input
that makes it to qmp_dispatch() and command responses, but not events.
Fine print on "input that makes it to qmp_dispatch()":
* You trace right before we execute the command, not when we receive,
parse and enqueue input.
* Corollary: input with certain errors is not traced.
* You don't trace the input text, you trace the unparsed parse tree.
All fine, I presume.
Existing tracepoints in monitor/qmp.c, and what information they send
(inessential bits omitted for clarity):
* handle_qmp_command
Handling a QMP command: unparsed parse tree
Fine print, safe to ignore:
- Out-of-band commands will be executed right away, in-band commands
will be queued. Tracepoints monitor_qmp_in_band_enqueue and
monitor_qmp_in_band_dequeue provide insight into that.
- This also receives and queues parse errors, without tracing them.
Tracepoint monitor_qmp_err_in_band traces them as they are dequeued.
* monitor_qmp_cmd_in_band
About to execute in-band command: command ID, if any
* monitor_qmp_cmd_out_of_band
About to execute out-of-band command: command ID, if any
* monitor_qmp_respond
About to send command response or event: QObject
For input, --trace qmp:* is like --trace handle_qmp_command, except it
traces late rather than early.
For output, --trace qmp:* is like --trace monitor_qmp_respond less
events.
The main improvement over existing tracepoints seems to be the ability
to filter on command names.
To get that, you overload the @name argument of QMP command
trace-event-set-state. In addition to the documented meaning "Event
name pattern", it also has an alternate, undocumented meaning "QMP
command name pattern". The "undocumented" part is easy enough to fix.
However, QMP heavily frowns on argument values that need to be parsed.
But before we discuss this in depth, we should decide whether we want
the filtering feature.
Management applications can enable and disable tracing as needed, but
doing it all in QEMU might be more convenient or robust.
Libvirt logs all QMP traffic. I doubt it'll make use of your filtering
feature. Cc'ing libvir-list just in case.
Another way to log all traffic is to route it through socat -x or
similar.
Opinions?