On Mon, Sep 12, 2011 at 4:33 PM, William Cohen <wco...@redhat.com> wrote: > The RHEL-6 version of qemu-kvm makes the tracepoints available to SystemTap. > I have been working on useful examples for the SystemTap tracepoints in qemu. > There doesn't seem to be a great number of examples showing the utility of > the tracepoints in diagnosing problems. However, I came across the following > blog entry that had several examples: > > http://blog.vmsplice.net/2011/03/how-to-write-trace-analysis-scripts-for.html > > I reimplemented the VirtqueueRequestTracker example from the blog in > SystemTap (the attached virtqueueleaks.stp). I can run it on RHEL-6's > qemu-kvm-0.12.1.2-2.160.el6_1.8.x86_64 and get output like the following. It > outputs the pid and the address of the elem that leaked when the script is > stopped like the following: > > $ stap virtqueueleaks.stp > ^C > pid elem > 19503 1c4af28 > 19503 1c56f88 > 19503 1c62fe8 > 19503 1c6f048 > 19503 1c7b0a8 > 19503 1c87108 > 19503 1c93168 > ... > > I am not that familiar with the internals of qemu. The script seems to > indicates qemu is leaking, but is that really the case? If there are > resource leaks, what output would help debug those leaks? What enhancements > can be done to this script to provide more useful information?
Leak tracing always has corner cases :). With virtio-blk this would indicate a leak because it uses a request-response model where the guest initiates I/O and the host responds. A guest that cleanly shuts down before you exit your SystemTap script should not leak requests for virtio-blk. With virtio-net the guest actually hands the host receive buffers and they are held until we can receive packets into them and return them to the host. We don't have a virtio_reset trace event, and due to this we're not accounting for clean shutdown (the guest driver resets the device to clear all virtqueues). I am submitting a patch to add virtio_reset() tracing. This will allow the script to delete all elements belonging to this virtio device. > Are there other examples of qemu probing people would like to see? The malloc/realloc/memalign/vmalloc/free/vfree trace events can be used for a few things: * Finding memory leaks. * Finding malloc/vfree or vmalloc/free mismatches. The rules are: malloc/realloc need free, memalign/vmalloc need vfree. They cannot be mixed. Stefan
