enter_state()
-->suspend_prepare(state);
-->suspend_freeze_processes();

static inline int suspend_freeze_processes(void)
{
	int error;

	error = freeze_processes();
	/*
	 * freeze_processes() automatically thaws every task if freezing
	 * fails. So we need not do anything extra upon error.
	 */
	if (error)
		return error;

	error = freeze_kernel_threads();
	/*
	 * freeze_kernel_threads() thaws only kernel threads upon freezing
	 * failure. So we have to thaw the userspace tasks ourselves.
	 */
	if (error)
		thaw_processes();

	return error;
}

Free_process is the thread of free userspace, and free_kernel_threads is the thread used for free kernel space

PM ﹣ freezing global variable indicates that userspace begins to enter the freeing stage. First, set the current process to suspend ﹣ task, which is responsible for freeing all other userspace tasks and calling try ﹣ to ﹣ free ﹣ tasks. In this function, suspend ﹣ task will always be in the while sleep. The exit conditions are as follows: once exiting, call that ﹣ process to re solve the free process freeze
1 (!todo || time_after(jiffies, end_time))，freeze_timeout_msecs = 20 * MSEC_PER_SEC , exit if the freeze action is not completed within 20s

2 (pm_wakeup_pending()) 

freeze_task The function will send a signal to the thread of user space, so that when the suspend task leaves the cpu, other threads will return from the kernel space and need to be processed __refrigerator

 

int freeze_processes(void)
{

	/* Make sure this task doesn't get frozen */
	current->flags |= PF_SUSPEND_TASK;

	pr_info("Freezing user space processes ... ");
	pm_freezing = true;
	error = try_to_freeze_tasks(true);
	/*
	 * Now that the whole userspace is frozen we need to disbale
	 * the OOM killer to disallow any further interference with
	 * killable tasks. There is no guarantee oom victims will
	 * ever reach a point they go away we have to wait with a timeout.
	 */
	if (error)
		thaw_processes();
	return error;
}


static int try_to_freeze_tasks(bool user_only)
{
	while (true) {
		todo = 0;
		for_each_process_thread(g, p) {
			if (p == current || !freeze_task(p))
				continue;
			if (!freezer_should_skip(p))
				todo++;
		}
		if (!user_only) {
			wq_busy = freeze_workqueues_busy();
			todo += wq_busy;
		}
		if (!todo || time_after(jiffies, end_time))
			break;

		if (pm_wakeup_pending()) {
			wakeup = true;
			break;
		}
		/*
		 * We need to retry, but first give the freezing tasks some
		 * time to enter the refrigerator.  Start with an initial
		 * 1 ms sleep followed by exponential backoff until 8 ms.
		 */
		usleep_range(sleep_usecs / 2, sleep_usecs);
		if (sleep_usecs < 8 * USEC_PER_MSEC)
			sleep_usecs *= 2;
	}
	if (todo) {
		pr_cont("\n");
		pr_err("Freezing of tasks %s after %d.%03d seconds "
		       "(%d tasks refusing to freeze, wq_busy=%d):\n",
		       wakeup ? "aborted" : "failed",
		       elapsed_msecs / 1000, elapsed_msecs % 1000,
		       todo - wq_busy, wq_busy);

		if (wq_busy)
			show_workqueue_state();

		if (!wakeup) {
			for_each_process_thread(g, p) {
				if (p != current && !freezer_should_skip(p)
				    && freezing(p) && !frozen(p))
					sched_show_task(p);
			}
		}
	} else {
		pr_cont("(elapsed %d.%03d seconds) ", elapsed_msecs / 1000,
			elapsed_msecs % 1000);
	}

	return todo ? -EBUSY : 0;
}