wxleong/lkm

A Linux Kernel Module Reference

The kernel module is designed to be non-monolithic with reusability in mind, comprising multiple layers:

• lkm: The primary entry point for the kernel module.
• chardev: This layer facilitates character-type device functionality.
• resource-manager: This layer enables concurrent access and orderly execution of shared hardware resources. The term "orderly" here refers to an operation being processed sequentially, with no overlapping allowed.
• realtime: This layer supports the execution of critical or time-sensitive operations that cannot be interrupted or preempted while also minimizing bus arbitration by hogging CPU cores.

Build:

$ make

Insert the kernel module:

$ sudo insmod mod.ko

Check if the kernel module has successfully initialized by verifying the presence of /dev/op0.

$ sudo ls /dev | grep op0

Run the sample userland application:

$ sudo ./test

Configure the kernel module using sysfs attribute groups:

$ ls -l /sys/class/eval/op0/realtime/
total 0
-rw-r--r-- 1 root root 4096 Sep  6 07:40 blocker_timeout
-rw-r--r-- 1 root root 4096 Sep  6 07:40 cpu_allowed
-r--r--r-- 1 root root 4096 Sep  6 07:40 cpu_max
-rw-r--r-- 1 root root 4096 Sep  6 07:40 worker_timeout

$ ls -l /sys/class/eval/op0/resource-manager/
total 0
-r--r--r-- 1 root root 4096 Sep  6 07:41 session_count
-rw-r--r-- 1 root root 4096 Sep  6 07:41 session_max

To read from an attribute:

$ cat /sys/class/eval/op0/realtime/cpu_max

To configure an attribute:

$ sudo sh -c "echo 1 > /sys/class/eval/op0/realtime/cpu_allowed"

Attribute Table:

Group	Attribute	Permission	Description
realtime	blocker_timeout	RW	The timeout value for a worker waiting for the blockers to enter the blocking mode.
realtime	worker_timeout	RW	The timeout value for the blockers waiting for the worker to complete its execution.
realtime	cpu_max	RO	Number of physical CPU core.
realtime	cpu_allowed	RW	<0 results in non-real-time execution. 0 allows execution to occur in a kthread running with a real-time scheduling policy (e.g., SCHED_FIFO). 1, in addition to 0, disables local interrupts but without hogging other CPU cores. >1, in addition to 0, hog other CPUs (and disables local interrupts) up to a maximum of cpu_max-1 cores.
resource-manager	session_max	RW	Maximum allowed sessions open simultaneously.
resource-manager	session_count	RO	Number of active sessions.

A worker is a single kthread where critical or time-sensitive operations are executed, whereas blockers are multiple kthreads used to hog CPU cores, preventing them from executing other tasks and wait for the worker to complete its execution.

'local interrupts' refers to interrupts that are associated with a specific CPU core.

lkm's Issues

Add put_device() after cdev_device_del()

Follow the best practice from https://www.kernel.org/doc/html/v5.6/driver-api/infrastructure.html#c.device_unregister