This tool allows profiling the CPU usage of a Rust kernel running in QEMU, using FlameGraphs. More specifically, this has been written for Maestro.

The repository contains the following components:

• a QEMU TCG plugin (written in C) for data acquisition (CPU profiling only)
• an aggregator (written in Rust) to convert the data into a form that can be processed by flamegraph.pl

The aggregator tool outputs one or several SVG file(s) with the desired FlameGraph(s).

Build

This section describes building the aggregator tool. CPU profiling also requires building the QEMU plugin.

First, make sure you FlameGraph is present:

git submodule init
git submodule update --init --recursive

Then, compile the aggregator:

cargo +nightly build --release

CPU Profiling

It works by sampling the execution of the code at a given rate. For each sample, the plugin collects the current callstack of the code being executed.

The assumption is made that more time the CPU spends executing a function, the higher is the probability the function shows up in the callstack.

Aggregating all the callstacks together allows building the FlameGraph.

Requirements

QEMU version 8.2.0 exactly is required (for CPU profiling only).

Note: another version of QEMU cannot be used because the API is not guaranteed to be consistent from one version to another

If compiling QEMU yourself, the option --enable-modules must be passed to ./configure.

Build

First, go in the plugin/ directory.

Build the QEMU plugin using:

QEMU_SRC=<path-to-QEMU-sources> make

Usage

First, make sure:

• The kernel is compiled with the option -Cforce-frame-pointers=yes on rustc
• Kernel symbols are present (not stripped)

Run QEMU with the plugin by adding the following argument (adapt parameters to your needs):

-plugin 'kern-profile.so,out=raw-data,delay=10'

Arguments:

• out is the path to the output file
• delay (optional) is the amount of microseconds between each sample

The output file can then be processed by the aggregator:

kern-profile raw-data <path-to-kernel-ELF>

Memory profiling

For memory profiling, the tool uses the data output by the memtrace feature of the Maestro kernel.

After collecting memtrace sample, you just have to run the aggregator tool:

kern-profile --alloc <path-to-memtrace-data> <path-to-kernel-ELF>

Caveats/missing features

The following issues need to be fixed in the future:

• Only one CPU core is supported
• Only x86 is supported
• Only the kernel can be profiled. It is not possible to load/observe several ELF at once (either kernel modules or userspace programs)