11.1. Scheduling Tasks

Very often, we have "housekeeping" tasks which have to be done at a certain time, or every so often. If the task is to be done by a process, we do it by putting it in the crontab file. If the task is to be done by a kernel module, we have two possibilities. The first is to put a process in the crontab file which will wake up the module by a system call when necessary, for example by opening a file. This is terribly inefficient, however -- we run a new process off of crontab, read a new executable to memory, and all this just to wake up a kernel module which is in memory anyway.

Instead of doing that, we can create a function that will be called once for every timer interrupt. The way we do this is we create a task, held in a workqueue_struct structure, which will hold a pointer to the function. Then, we use queue_delayed_work to put that task on a task list called my_workqueue, which is the list of tasks to be executed on the next timer interrupt. Because we want the function to keep on being executed, we need to put it back on my_workqueue whenever it is called, for the next timer interrupt.

There's one more point we need to remember here. When a module is removed by rmmod, first its reference count is checked. If it is zero, module_cleanup is called. Then, the module is removed from memory with all its functions. Things need to be shut down properly, or bad things will happen. See the code below how this can be done in a safe way.

Example 11-1. sched.c

/*
 *  sched.c - scheduale a function to be called on every timer interrupt.
 *
 *  Copyright (C) 2001 by Peter Jay Salzman
 */

/* 
 * The necessary header files 
 */

/* 
 * Standard in kernel modules 
 */
#include <linux/kernel.h>       /* We're doing kernel work */
#include <linux/module.h>       /* Specifically, a module */
#include <linux/proc_fs.h>      /* Necessary because we use the proc fs */
#include <linux/workqueue.h>    /* We scheduale tasks here */
#include <linux/sched.h>        /* We need to put ourselves to sleep 
                                   and wake up later */
#include <linux/init.h>         /* For __init and __exit */
#include <linux/interrupt.h>    /* For irqreturn_t */

struct proc_dir_entry *Our_Proc_File;
#define PROC_ENTRY_FILENAME "sched"
#define MY_WORK_QUEUE_NAME "WQsched.c"

/* 
 * The number of times the timer interrupt has been called so far 
 */
static int TimerIntrpt = 0;

static void intrpt_routine(void *);

static int die = 0;             /* set this to 1 for shutdown */

/* 
 * The work queue structure for this task, from workqueue.h 
 */
static struct workqueue_struct *my_workqueue;

static struct work_struct Task;
static DECLARE_WORK(Task, intrpt_routine, NULL);

/* 
 * This function will be called on every timer interrupt. Notice the void*
 * pointer - task functions can be used for more than one purpose, each time
 * getting a different parameter.
 */
static void intrpt_routine(void *irrelevant)
{
        /* 
         * Increment the counter 
         */
        TimerIntrpt++;

        /* 
         * If cleanup wants us to die
         */
        if (die == 0)
                queue_delayed_work(my_workqueue, &Task, 100);
}

/* 
 * Put data into the proc fs file. 
 */
ssize_t
procfile_read(char *buffer,
              char **buffer_location,
              off_t offset, int buffer_length, int *eof, void *data)
{
        int len;                /* The number of bytes actually used */

        /* 
         * It's static so it will still be in memory 
         * when we leave this function
         */
        static char my_buffer[80];

        /* 
         * We give all of our information in one go, so if anybody asks us
         * if we have more information the answer should always be no.
         */
        if (offset > 0)
                return 0;

        /* 
         * Fill the buffer and get its length 
         */
        len = sprintf(my_buffer, "Timer called %d times so far\n", TimerIntrpt);

        /* 
         * Tell the function which called us where the buffer is 
         */
        *buffer_location = my_buffer;

        /* 
         * Return the length 
         */
        return len;
}

/* 
 * Initialize the module - register the proc file 
 */
int __init init_module()
{
        /*
         * Create our /proc file
         */
        Our_Proc_File = create_proc_entry(PROC_ENTRY_FILENAME, 0644, NULL);
        
        if (Our_Proc_File == NULL) {
                remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
                printk(KERN_ALERT "Error: Could not initialize /proc/%s\n",
                       PROC_ENTRY_FILENAME);
                return -ENOMEM;
        }

        Our_Proc_File->read_proc = procfile_read;
        Our_Proc_File->owner = THIS_MODULE;
        Our_Proc_File->mode = S_IFREG | S_IRUGO;
        Our_Proc_File->uid = 0;
        Our_Proc_File->gid = 0;
        Our_Proc_File->size = 80;

        /* 
         * Put the task in the work_timer task queue, so it will be executed at
         * next timer interrupt
         */
        my_workqueue = create_workqueue(MY_WORK_QUEUE_NAME);
        queue_delayed_work(my_workqueue, &Task, 100);
        
        
        printk(KERN_INFO "/proc/%s created\n", PROC_ENTRY_FILENAME);
        
        return 0;
}

/* 
 * Cleanup
 */
void __exit cleanup_module()
{
        /* 
         * Unregister our /proc file 
         */
        remove_proc_entry(PROC_ENTRY_FILENAME, &proc_root);
        printk(KERN_INFO "/proc/%s removed\n", PROC_ENTRY_FILENAME);

        die = 1;                /* keep intrp_routine from queueing itself */
        cancel_delayed_work(&Task); /* no "new ones" */
        flush_workqueue(my_workqueue);  /* wait till all "old ones" finished */
        destroy_workqueue(my_workqueue);

        /* 
         * Sleep until intrpt_routine is called one last time. This is 
         * necessary, because otherwise we'll deallocate the memory holding 
         * intrpt_routine and Task while work_timer still references them.
         * Notice that here we don't allow signals to interrupt us.
         *
         * Since WaitQ is now not NULL, this automatically tells the interrupt
         * routine it's time to die.
         */

}

/* 
 * some work_queue related functions
 * are just available to GPL licensed Modules 
 */
MODULE_LICENSE("GPL");