Re: [libvirt] RFC: Libvirt extensions for QEMU tracing

On Tue, Oct 26, 2010 at 12:15:30PM +0530, Prerna Saxena wrote: > Hi, > Tracing infrastructure is now a part of upstream QEMU, which allows > options to choose different trace backends to handle trace-events of > interest. The choice of 'simple' trace backend allows users to > dynamically enable/disable trace events for a running qemu instance as > well as to set options for logging traces to a desired file via the qemu > monitor. > While the QMP interfaces are still under discussion > (http://www.mail-archive.com/qemu-devel@nongnu.org/msg44535.html), I > propose the following extensions for virsh to make use of human-monitor > tracing commands: > > 1. virsh trace-events DOMAIN-ID [set TRACE-EVENT ON ] > ---------------------------------------------------- > - Implements QEMU monitor command 'trace event' to change state of a > particular trace-event. > - Eg, virsh trace-events DOMAIN-ID set ABC on > : changes state of trace-event 'ABC' to enabled. > - When specified without arguments, it implements QEMU monitor > command to show all currently available trace events and their state for a > specific instance. > - Eg, virsh trace-events DOMAIN-ID > : lists all trace-events with their state for that instance. > > 2. virsh trace-file DOMAIN_ID [set FILENAME | --enable | --disable] > ------------------------------------------------------------------- > - Implements the qemu monitor command : trace-file > - Without any arguments, it lists the currently active trace output > file with its state. > - The 'set' subcommand changes the output file to FILENAME. > - The --enable and --disable switches respectively enable and > disable writing of trace data to output file. > > The catch is, these are only available for 'simple' trace backend for > qemu, so one would need to be careful of handling failures when these > commands are passed to qemu instances compiled with a different trace > backend. As such I'm not really convinced this is going to be useful. I don't really see distros choosing to build the simple trace backend, when the other backends (LTT-NG, or soon DTrace) are so much more flexible and functional. Certainly in both Fedora and RHEL we'd just go for a DTrace/SystemTAP tracing backend because that gives you end-to-end tracing across the entire stack (virt-manager, libvirt, qemu/kvm and kernel), as opposed to an isolated QEMU specific tool.