Workqueue

  • In this program, you are going to learn

  • How to create workqueue ?

  • How to allocate the work to the queue ?

Topics in this section,

  • DECLARE_WORK creates a workqueue by the name and the function that we are passing in the second argument gets scheduled in the queue. For example,

DECLARE_WORK(workqueue, workqueue_func);

  • schedule_work used to allocate the work to the queue. For example,

schedule_work(&workqueue);

  • Here is the explanation of the source code.

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/workqueue.h>

  • Include the headers file to refer the APIs used in this program.

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux_usr");
MODULE_DESCRIPTION("Workqueue");

  • Add the modules macro which contains the information of module such as license, author and description.

void workqueue_func(struct work_struct *work);

DECLARE_WORK(workqueue, workqueue_func);

  • Initialize the variables and APIs which is used in this program.

  • DECLARE_WORK is used to declare the workqueue.

static int __init queue_init(void)
{
    pr_info("Hello, world\n");
    schedule_work(&workqueue);

    return 0;
}

  • Create the module init function which will be called once the module is loaded into linux kernel.

  • schedule_work is used to schedule the workqueue.

static void __exit queue_exit(void)
{
    pr_info("Goodbye, world\n");
}

  • Create the module exit function which will be called once the module is unloaded into linux kernel.

void workqueue_func(struct work_struct *work)
{
    pr_info("Inside Workqueue function\n");
}

  • Workqueue APIs is called once the work is scheduled.

  • This API will be passed as a parameter during the time of declaring the workqueue.

module_init(queue_init);
module_exit(queue_exit);

  • Mention the module init and exit functions which needs to be executed when the module is loaded and unloaded.

 1#include <linux/init.h>
 2#include <linux/kernel.h>
 3#include <linux/module.h>
 4#include <linux/fs.h>
 5#include <linux/workqueue.h>
 6
 7MODULE_LICENSE("GPL");
 8MODULE_AUTHOR("Linux_usr");
 9MODULE_DESCRIPTION("Workqueue");
10
11void workqueue_func(struct work_struct *work);
12
13DECLARE_WORK(workqueue, workqueue_func);
14
15void workqueue_func(struct work_struct *work)
16{
17	pr_info("Inside Workqueue function\n");
18}
19
20
21static int __init queue_init(void)
22{
23	pr_info("Hello, world\n");
24	schedule_work(&workqueue);
25
26	return 0;
27}
28
29static void __exit queue_exit(void)
30{
31	pr_info("Goodbye, world\n");
32}
33
34module_init(queue_init);
35module_exit(queue_exit);

 1obj-m += work_queue.o
 2 
 3KDIR = /lib/modules/$(shell uname -r)/build
 4 
 5 
 6all:
 7	make -C $(KDIR)  M=$(shell pwd) modules
 8 
 9clean:
10	make -C $(KDIR)  M=$(shell pwd) clean

  • Run make to load the kernel module.

$ make
make -C /lib/modules/5.4.0-150-generic/build M=$HOME/kernel_work_queue modules
make[1]: Entering directory '/usr/src/linux-headers-5.4.0-150-generic'
  CC [M]  $HOME/kernel_work_queue/work_queue.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC [M]  $HOME/kernel_work_queue/work_queue.mod.o
  LD [M]  $HOME/kernel_work_queue/work_queue.ko
make[1]: Leaving directory '/usr/src/linux-headers-5.4.0-150-generic'

  • Check if work_queue.ko is generated or not using ls command.

$ ls -l
total 36
-rw-rw-r-- 1 test test  154 Feb 26 13:18 Makefile
-rw-rw-r-- 1 test test   47 Feb 26 13:18 modules.order
-rw-rw-r-- 1 test test    0 Feb 26 13:18 Module.symvers
-rw-rw-r-- 1 test test  820 Feb 26 13:18 work_queue.c
-rw-rw-r-- 1 test test 5880 Feb 26 13:18 work_queue.ko
-rw-rw-r-- 1 test test   47 Feb 26 13:18 work_queue.mod
-rw-rw-r-- 1 test test  919 Feb 26 13:18 work_queue.mod.c
-rw-rw-r-- 1 test test 3448 Feb 26 13:18 work_queue.mod.o
-rw-rw-r-- 1 test test 3320 Feb 26 13:18 work_queue.o

  • Load the module to kernel using insmod command.

$ sudo insmod ./work_queue.ko

  • Check kernel messages to verify if the module is loaded or not.

$ dmesg
[ 1018.983824] Hello, world
[ 1018.983953] Inside Workqueue function

  • Unload the module using rmmod command.

$ sudo rmmod work_queue

  • Check kernel messages to see if the module is unloaded or not.

$ dmesg
[ 1018.983824] Hello, world
[ 1018.983953] Inside Workqueue function
[ 1027.144246] Goodbye, world

API

Learning

DECLARE_WORK

To create the workqueue

schedule_work

To allocate the work to the queue