PM / freezer: Don't get over-anxious while waiting

Freezing isn't exactly the most latency sensitive operation and
there's no reason to burn cpu cycles and power waiting for it to
complete.  msleep(10) instead of yield().  This should improve
reliability of emergency hibernation.

[rjw: Modified the comment next to the msleep(10).]
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>

kernel/power/process.c

@@ -14,6 +14,7 @@
 #include <linux/module.h>
 #include <linux/syscalls.h>
 #include <linux/freezer.h>
+#include <linux/delay.h>
 
 /* 
  * Timeout for stopping processes
@@ -41,7 +42,7 @@ static int try_to_freeze_tasks(bool sig_only)
 	do_gettimeofday(&start);
 
 	end_time = jiffies + TIMEOUT;
-	do {
+	while (true) {
 		todo = 0;
 		read_lock(&tasklist_lock);
 		do_each_thread(g, p) {
@@ -62,10 +63,15 @@ static int try_to_freeze_tasks(bool sig_only)
 				todo++;
 		} while_each_thread(g, p);
 		read_unlock(&tasklist_lock);
-		yield();			/* Yield is okay here */
-		if (time_after(jiffies, end_time))
+		if (!todo || time_after(jiffies, end_time))
 			break;
-	} while (todo);
+
+		/*
+		 * We need to retry, but first give the freezing tasks some
+		 * time to enter the regrigerator.
+		 */
+		msleep(10);
+	}
 
 	do_gettimeofday(&end);
 	elapsed_csecs64 = timeval_to_ns(&end) - timeval_to_ns(&start);