clocksource: prevent selection of low resolution clocksourse also for nohz=on

commit 3f68535adad (clocksource: sanity check sysfs clocksource
changes) prevents selection of non high resolution capable
clocksources when high resolution mode is active, but did not take
into account that the same rules apply for highres=off nohz=on.

Check the tick device mode instead of hrtimer_hres_active() to verify
whether the system needs to be protected from a switch to jiffies or
other non highres capable clock sources.

Reported-by: Luming Yu <luming.yu@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 27 insertions, 8 deletions

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 18b9f5da4ee9..592bf584d1d2 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -30,7 +30,6 @@
 #include <linux/module.h>
 #include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */
 #include <linux/tick.h>
-#include <linux/hrtimer.h>
 
 void timecounter_init(struct timecounter *tc,
                       const struct cyclecounter *cc,
@@ -511,13 +510,13 @@ static ssize_t sysfs_override_clocksource(struct sys_device *dev,
         }
 
         /*
-         * Check to make sure we don't switch to a non-HRT usable
-         * clocksource if HRT is enabled and running
+         * 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 (hrtimer_hres_active() &&
+        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 mode\n", ovr->name);
+                        "Cannot switch while in HRT/NOHZ mode\n", ovr->name);
                 ovr = NULL;
                 override_name[0] = 0;
         }
@@ -550,9 +549,12 @@ sysfs_show_available_clocksources(struct sys_device *dev,
 
         spin_lock_irq(&clocksource_lock);
         list_for_each_entry(src, &clocksource_list, list) {
-                /* Don't show non-HRES clocksource if HRES is enabled */
-                if (!hrtimer_hres_active() ||
-                                (src->flags & CLOCK_SOURCE_VALID_FOR_HRES))
+                /*
+                 * 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);
diff --git a/kernel/time/tick-oneshot.c b/kernel/time/tick-oneshot.c
index 2e8de678e767..a96c0e2b89cf 100644
--- a/kernel/time/tick-oneshot.c
+++ b/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