Linux Kernel Testing and Debugging

Linux PM Sub-system Testing in Simulation Mode

The Linux PM sub-system provides five PM test modes to test hibernation in a simulated mode. These modes allow exercising the hibernation code in various layers of the kernel without actually suspending the system. This is useful when there is a concern that suspend might not work on a specific platform and help detect errors in a simulation similar to simulating flying a plane, so to speak.

freezer - test the freezing of processes
echo freezer > /sys/power/pm_test
echo platform > /sys/power/disk
echo disk > /sys/power/state
devices - test the freezing of processes and suspending of devices
echo devices > /sys/power/pm_test
echo platform > /sys/power/disk
echo disk > /sys/power/state
platform - test the freezing of processes, suspending of devices and platform global control methods(*)
echo platform > /sys/power/pm_test
echo platform > /sys/power/disk
echo disk > /sys/power/state
processors - test the freezing of processes, suspending of devices, platform global control methods(*) and the disabling of non-boot CPUs
echo processors > /sys/power/pm_test
echo platform > /sys/power/disk
echo disk > /sys/power/state
core - test the freezing of processes, suspending of devices, platform global control methods, the disabling of non-boot CPUs and suspending of platform/system devices. Note: this mode is tested on ACPI systems.
echo core > /sys/power/pm_test
echo platform > /sys/power/disk
echo disk > /sys/power/state

Linux PM Sub-system Trace Events

PM sub-system supports several tracepoints and trace events that can be enabled to trigger during run-time. I will give an overview on how to enable couple of these trace events and where to find the trace information they generate:

Enabling PM events at run-time:
cd /sys/kernel/debug/tracing/events/power
echo 1 > cpu_frequency/enable
cat /sys/kernel/debug/tracing/set_event
less /sys/kernel/debug/tracing/trace
Enabling events at boot-time kernel trace parameter with a kernel boot option:

For more information on Linux PM testing, please consult the Documentation/power/basic-pm-debugging.txt and other documents in the same directory.

git bisect

git bisect is an invaluable and powerful tool to isolate an offending commit. I will go over very basic git bisect steps.

This is how the process works:
git bisect start
git bisect bad	 # Current version is bad
git bisect good v3.14-rc6	# last good version

Once, one bad and one good version are specified, git bisect will start bisecting by pulling in commits between the good version and the bad. Once a set of commits are pulled in, compile the kernel, install, test, and tag the version good or bad. This process repeats until the selected commits are tested and tagged as good or bad. There can be several kernel versions to test. When the last version is tested, git bisect will flag a commit that is bad. The following useful git-bisect command can aid in using git-bisect process:

See step by step bisect progress
git bisect log
Reset git bisect can be used in case of mistakes in tagging, save git log output and replay prior to reset
git bisect reset
Replay a git-bisect log
git bisect replay git_log_output

git bisect can be run on a section of kernel source tree if the problem is clearly in that area. For example, when debugging a problem in radeon driver, running git bisect on drivers/drm/radeon will limit the scope of bisect to just the commits to drivers/drm/radeon driver.

Start git bisect on a section of a kernel tree
git bisect start drivers/drm/radeon


Shuah Khan is a Senior Linux Kernel Developer at Samsung's Open Source Group.