Merge branch 'timers-for-linus-clocksource' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

* 'timers-for-linus-clocksource' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
  clocksource: prevent selection of low resolution clocksourse also for nohz=on
  clocksource: sanity check sysfs clocksource changes

include/linux/tick.h

@@ -97,10 +97,12 @@ extern void tick_clock_notify(void);
 extern int tick_check_oneshot_change(int allow_nohz);
 extern struct tick_sched *tick_get_tick_sched(int cpu);
 extern void tick_check_idle(int cpu);
+extern int tick_oneshot_mode_active(void);
 # else
 static inline void tick_clock_notify(void) { }
 static inline int tick_check_oneshot_change(int allow_nohz) { return 0; }
 static inline void tick_check_idle(int cpu) { }
+static inline int tick_oneshot_mode_active(void) { return 0; }
 # endif
 
 #else /* CONFIG_GENERIC_CLOCKEVENTS */
@@ -109,6 +111,7 @@ static inline void tick_cancel_sched_timer(int cpu) { }
 static inline void tick_clock_notify(void) { }
 static inline int tick_check_oneshot_change(int allow_nohz) { return 0; }
 static inline void tick_check_idle(int cpu) { }
+static inline int tick_oneshot_mode_active(void) { return 0; }
 #endif /* !CONFIG_GENERIC_CLOCKEVENTS */
 
 # ifdef CONFIG_NO_HZ

kernel/time/clocksource.c

@@ -509,6 +509,18 @@ static ssize_t sysfs_override_clocksource(struct sys_device *dev,
 		}
 	}
 
+	/*
+	 * Check to make sure we don't switch to a non-highres capable
+	 * clocksource if the tick code is in oneshot mode (highres or nohz)
+	 */
+	if (tick_oneshot_mode_active() &&
+	    !(ovr->flags & CLOCK_SOURCE_VALID_FOR_HRES)) {
+		printk(KERN_WARNING "%s clocksource is not HRT compatible. "
+			"Cannot switch while in HRT/NOHZ mode\n", ovr->name);
+		ovr = NULL;
+		override_name[0] = 0;
+	}
+
 	/* Reselect, when the override name has changed */
 	if (ovr != clocksource_override) {
 		clocksource_override = ovr;
@@ -537,7 +549,13 @@ sysfs_show_available_clocksources(struct sys_device *dev,
 
 	spin_lock_irq(&clocksource_lock);
 	list_for_each_entry(src, &clocksource_list, list) {
-		count += snprintf(buf + count,
+		/*
+		 * Don't show non-HRES clocksource if the tick code is
+		 * in one shot mode (highres=on or nohz=on)
+		 */
+		if (!tick_oneshot_mode_active() ||
+		    (src->flags & CLOCK_SOURCE_VALID_FOR_HRES))
+			count += snprintf(buf + count,
 				  max((ssize_t)PAGE_SIZE - count, (ssize_t)0),
 				  "%s ", src->name);
 	}

kernel/time/tick-oneshot.c

@@ -128,6 +128,23 @@ int tick_switch_to_oneshot(void (*handler)(struct clock_event_device *))
 	return 0;
 }
 
+/**
+ * tick_check_oneshot_mode - check whether the system is in oneshot mode
+ *
+ * returns 1 when either nohz or highres are enabled. otherwise 0.
+ */
+int tick_oneshot_mode_active(void)
+{
+	unsigned long flags;
+	int ret;
+
+	local_irq_save(flags);
+	ret = __get_cpu_var(tick_cpu_device).mode == TICKDEV_MODE_ONESHOT;
+	local_irq_restore(flags);
+
+	return ret;
+}
+
 #ifdef CONFIG_HIGH_RES_TIMERS
 /**
  * tick_init_highres - switch to high resolution mode