libvirt/docs/kbase/systemtap.rst
Han Han 51f7fdade2 kbase: Add knowledge base for libvirt systemtap
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Han Han <hhan@redhat.com>
2020-08-21 11:34:23 +01:00

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====================
Systemtap of Libvirt
====================
.. contents::
`Systemtap <https://sourceware.org/systemtap/>`__ is a scripting
language and tool for dynamically probing or tracing in Linux kernel
space or user space. This page is about the usage of systemtap
in libvirt tracing.
Preparation
===========
Libvirt
-------
Libvirt should be configured with the systemtap option to support libvirt
probing events in systemtap.
For libvirt before **6.7.0**, it can be configured by:
::
mkdir build
cd build
../configure --with-dtrace
For libvirt **6.7.0** or later, configure it by the ``meson`` (seeing
`libvirt compiling <https://libvirt.org/compiling.html>`__):
::
meson build -Ddtrace=enabled
For the libvirt binaries installed by the package manager like ``dnf`` or
``apt``, if libvirt systemtap tapset ``/usr/share/systemtap/tapset/libvirt_*``
exists, it means the libvirt enables the systemtap feature.
Systemtap
---------
For most of linux distributions, execute ``stap-prep`` by root to prepare the
environment for systemtap after installing the systemtap. If your distribution
doesn't have ``stap-prep``, install the ``kernel debuginfo`` packages manually.
After these above, run this test command to confirm the systemtap works well:
::
stap -e 'probe oneshot{ printf("hello world\n")}'
Tracing events
==============
The libvirt systemtap tracing events are defined in tapset
``/usr/share/systemtap/tapset/libvirt_*``. Libvirt support these type of tracing
events: ``dbus``, ``event_glib``, ``object``, ``qemu``, ``rpc``.
List all tracing events in libvirt:
::
grep 'probe libvirt.[a-z_0-9.]*' /usr/share/systemtap/tapset/libvirt_* -o|cut -f 2 -d :
Tracing examples
================
Here is an example of the systemtap script to trace the QMP messages sent from libvirtd
daemon to the qemu process.
``qmp.stp``:
::
probe begin
{
printf("Start tracing\n")
}
probe libvirt.qemu.monitor_send_msg
{
printf("QMPs: %s", msg);
}
Then run the systemtap script attaching to the libvirtd process:
::
stap qmp.stp -x `pidof libvirtd`
To trace a libvirtd started from command line, use the option ``-c``
::
stap qmp.stp -c "/usr/sbin/libvirtd"
Then after seeing the welcome message "Start tracing" from systemtap, then execute a virsh
command associated with QMP, for example ``virsh domstats``. Then get the QMP tracing logs
from systemtap. For example, the result from ``virsh domstats``
::
QMPs: {"execute":"query-balloon","id":"libvirt-393"}
QMPs: {"execute":"qom-get","arguments":{"path":"/machine/peripheral/balloon0","property":"guest-stats"},"id":"libvirt-394"}
QMPs: {"execute":"query-blockstats","id":"libvirt-395"}
QMPs: {"execute":"query-named-block-nodes","id":"libvirt-396"}
QMPs: {"execute":"query-iothreads","id":"libvirt-397"}
For more examples of libvirt systemtap scripts, see the scripts in ``/usr/share/doc/libvirt-docs/examples/systemtap``
For more details of systemtap language, see `document of systemtap <https://sourceware.org/systemtap/documentation.html>`__