How to sleep in the Linux kernel?

I needed to include <linux/delay.h> to use msleep in kernel space.

Linux kernel documentation

The Linux kernel documentation under Documentation/timers/timers-howto.txt has a good rundown of the main methods:

Inserting Delays----------------The first, and most important, question you need to ask is "Is mycode in an atomic context?"  This should be followed closely by "Doesit really need to delay in atomic context?" If so...ATOMIC CONTEXT:    You must use the *delay family of functions. These    functions use the jiffie estimation of clock speed    and will busy wait for enough loop cycles to achieve    the desired delay:    ndelay(unsigned long nsecs)    udelay(unsigned long usecs)    mdelay(unsigned long msecs)    udelay is the generally preferred API; ndelay-level    precision may not actually exist on many non-PC devices.    mdelay is macro wrapper around udelay, to account for    possible overflow when passing large arguments to udelay.    In general, use of mdelay is discouraged and code should    be refactored to allow for the use of msleep.NON-ATOMIC CONTEXT:    You should use the *sleep[_range] family of functions.    There are a few more options here, while any of them may    work correctly, using the "right" sleep function will    help the scheduler, power management, and just make your    driver better :)    -- Backed by busy-wait loop:        udelay(unsigned long usecs)    -- Backed by hrtimers:        usleep_range(unsigned long min, unsigned long max)    -- Backed by jiffies / legacy_timers        msleep(unsigned long msecs)        msleep_interruptible(unsigned long msecs)    Unlike the *delay family, the underlying mechanism    driving each of these calls varies, thus there are    quirks you should be aware of.    SLEEPING FOR "A FEW" USECS ( < ~10us? ):        * Use udelay        - Why not usleep?            On slower systems, (embedded, OR perhaps a speed-            stepped PC!) the overhead of setting up the hrtimers            for usleep *may* not be worth it. Such an evaluation            will obviously depend on your specific situation, but            it is something to be aware of.    SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):        * Use usleep_range        - Why not msleep for (1ms - 20ms)?            Explained originally here:                http://lkml.org/lkml/2007/8/3/250            msleep(1~20) may not do what the caller intends, and            will often sleep longer (~20 ms actual sleep for any            value given in the 1~20ms range). In many cases this            is not the desired behavior.        - Why is there no "usleep" / What is a good range?            Since usleep_range is built on top of hrtimers, the            wakeup will be very precise (ish), thus a simple            usleep function would likely introduce a large number            of undesired interrupts.            With the introduction of a range, the scheduler is            free to coalesce your wakeup with any other wakeup            that may have happened for other reasons, or at the            worst case, fire an interrupt for your upper bound.            The larger a range you supply, the greater a chance            that you will not trigger an interrupt; this should            be balanced with what is an acceptable upper bound on            delay / performance for your specific code path. Exact            tolerances here are very situation specific, thus it            is left to the caller to determine a reasonable range.    SLEEPING FOR LARGER MSECS ( 10ms+ )        * Use msleep or possibly msleep_interruptible        - What's the difference?            msleep sets the current task to TASK_UNINTERRUPTIBLE            whereas msleep_interruptible sets the current task to            TASK_INTERRUPTIBLE before scheduling the sleep. In            short, the difference is whether the sleep can be ended            early by a signal. In general, just use msleep unless            you know you have a need for the interrupt

Adapted from this awesome answer: https://stackoverflow.com/a/39921020/895245

Next have a look at the documentation comment on each of those functions in the source. E.g.: usleep_range:

/** * usleep_range - Sleep for an approximate time * @min: Minimum time in usecs to sleep * @max: Maximum time in usecs to sleep * * In non-atomic context where the exact wakeup time is flexible, use * usleep_range() instead of udelay().  The sleep improves responsiveness * by avoiding the CPU-hogging busy-wait of udelay(), and the range reduces * power usage by allowing hrtimers to take advantage of an already- * scheduled interrupt instead of scheduling a new one just for this sleep. */void __sched usleep_range(unsigned long min, unsigned long max)

LDD3 7.3. Delaying Executionis another must-have resource.

Minimal runnable examples

Finally write your own minimal tests to try them out!

#include <linux/delay.h> /* usleep_range */#include <linux/kernel.h>#include <linux/kthread.h>#include <linux/module.h>MODULE_LICENSE("GPL");static struct task_struct *kthread;static int work_func(void *data){    int i = 0;    while (!kthread_should_stop()) {        printk(KERN_INFO "%d\n", i);        usleep_range(1000000, 1000001);        i++;        if (i == 10)            i = 0;    }    return 0;}int init_module(void){    kthread = kthread_create(work_func, NULL, "mykthread");    wake_up_process(kthread);    return 0;}void cleanup_module(void){    /* Waits for thread to return. */    kthread_stop(kthread);}

GitHub upsteram.