Workqueue : queue_work: periodic

Workqueue : queue_work: periodic

• In this program, you are going to learn

• How to schedule a specific workqueue periodically ?

• How to use the below API’s ?

  • INIT_WORK

  • queue_work

  • cancel_work_sync

Topics in this section,

• Key learnings in this program

• Explanation of program part by part

  • List of headers

  • Module macros

  • Initialize variables and functions

  • Module init function

  • Module exit function

  • Workqueue APIs

  • Mention init and exit functions

• See full program below

• Makefile

• Compile and Load

• Summary of workqueue APIs

Key learnings in this program

• queue_work is a function used to schedule a work item for execution on a specific workqueue. For example,

queue_work(system_wq, &q_work);

• cancel_work_sync is a function used to cancel a scheduled work item and wait for it to complete before returning. For example,

cancel_work_sync(&q_work);

• To schedule a specific workqueue periodically, we need to call queue_work inside the workqueue_func.

static void workqueue_func(struct work_struct * work)
{
        pr_info("Inside Workqueue function\n");
        queue_work(system_wq, &q_work);
}

Explanation of program part by part

• Here is the explanation of the source code.

List of headers

#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.

Modules macro

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.

Initialize variables and functions

void workqueue_func(struct work_struct *work);

static struct work_struct q_work;

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

Module init function

static int __init queue_init(void)
{
    INIT_WORK(&q_work, workqueue_func);
    queue_work(system_wq, &q_work);

    pr_info("Driver Loaded\n");

    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.

Module exit function

static void __exit queue_exit(void)
{
    cancel_work_sync(&q_work);
        pr_info("Driver Unloaded...\n");
}

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

Workqueue APIs

void workqueue_func(struct work_struct *work)
{
    pr_info("Inside Workqueue function\n");
    queue_work(system_wq, &q_work);
}

• Workqueue APIs is called once the work is scheduled.

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

Mention init and exit functions

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.

See the full program below

work_queue.c

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

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

static void workqueue_func(struct work_struct *work);

static struct work_struct q_work;

static void workqueue_func(struct work_struct *work)
{
	pr_info("Inside Workqueue function\n");
	queue_work(system_wq, &q_work);
}

static int __init queue_init(void)
{
	INIT_WORK(&q_work, workqueue_func);
	queue_work(system_wq, &q_work);

	pr_info("Driver Loaded...\n");

	return 0;
}

static void __exit queue_exit(void)
{
	cancel_work_sync(&q_work);
	pr_info("Driver Unloaded...\n");
}

module_init(queue_init);
module_exit(queue_exit);

Makefile

obj-m += work_queue.o
 
KDIR = /lib/modules/$(shell uname -r)/build
 
 
all:
	make -C $(KDIR)  M=$(shell pwd) modules
 
clean:
	make -C $(KDIR)  M=$(shell pwd) clean

Compile and Load

• 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
[79276.652478] Inside Workqueue function
[79276.652479] Inside Workqueue function
[79276.652479] Inside Workqueue function
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• Unload the module using rmmod command.

$ sudo rmmod work_queue

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

$ dmesg
[79276.652478] Inside Workqueue function
[79276.652479] Inside Workqueue function
[79276.652479] Inside Workqueue function
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[79276.666072] Driver Unloaded...

Summary of workqueue APIs

API	Learning
INIT_WORK	Used for initializing a workqueue structure
queue_work	To schedule a workqueue
cancel_work_sync	To cancel a scheduled workqueue