Understand jiffy updates in queues at work

Programming
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1

Investigating the Effects of Workqueue on Sleep Duration and Jiffy Count

This investigation looks into the effects of using a workqueue on sleep duration and jiffy count.

Kernel Module

The following kernel module is used:

#include <linux/module.h>
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/time.h>
#include <linux/delay.h>
#include <linux/workqueue.h>
#include <linux/jiffies.h>

static DECLARE_WAIT_QUEUE_HEAD(my_wq);
static int condition = 0;

/* declare a work queue*/
static struct work_struct wrk;

static void work_handler(struct work_struct *work)
{ 
    pr_info("Waitqueue module handler %s, ms cnt is %u\n", __FUNCTION__, jiffies_to_msecs(jiffies));
    msleep(3000);
    pr_info("Wake up the sleeping module\n");
    condition = 1;
    wake_up_interruptible(&my_wq);
}

static int __init my_init(void)
{
    pr_info("Wait queue example, ms count is %u\n", jiffies_to_msecs(jiffies));

    INIT_WORK(&wrk, work_handler);
    schedule_work(&wrk);

    pr_info("Going to sleep %s\n", __FUNCTION__);
    wait_event_interruptible(my_wq, condition != 0);

    pr_info("woken up by the work job at %ums\n", jiffies_to_msecs(jiffies));
    return 0;
}

void my_exit(void)
{
    pr_info("waitqueue example cleanup\n");
}

module_init(my_init);
module_exit(my_exit);
MODULE_AUTHOR("Aijaz Baig <aijazbaig1@gmail.com>");
MODULE_LICENSE("GPL");

Results

Output from running the module:

  • First run:

    Jan  3 06:53:27 buildroot user.info kernel: Wait queue example, ms count is 6144890
Jan  3 06:53:27 buildroot user.info kernel: Going to sleep my_init
Jan  3 06:53:27 buildroot user.info kernel: Waitqueue module handler work_handler, ms cnt is 6144890
Jan  3 06:53:30 buildroot user.info kernel: Wake up the sleeping module
Jan  3 06:53:30 buildroot user.info kernel: woken up by the work job at 6147900ms

  • Second run:

    Jan  3 06:59:28 buildroot user.info kernel: Wait queue example, ms count is 6506580
Jan  3 06:59:28 buildroot user.info kernel: Going to sleep my_init
Jan  3 06:59:29 buildroot user.info kernel: Waitqueue module handler work_handler, ms cnt is 6506640
Jan  3 06:59:31 buildroot user.info kernel: Wake up the sleeping module
Jan  3 06:59:31 buildroot user.info kernel: woken up by the work job at 6509660ms

Analysis

Two noticeable things can be observed:

  1. The actual sleep duration is different from the one intended (a difference of ~15ms -20ms).
  2. The jiffy count under the __init function and the work_handler() function is different for the second run.

This suggests that using a workqueue can have an effect on sleep duration and jiffy count.

2

Using a workqueue can affect sleep duration and jiffy count. This is evident from the output of the kernel module provided in the investigation, where the actual sleep duration is different from the intended duration by ~15ms -20ms, and the jiffy count under the __init function and the work_handler() function is different for the second run.