5 files changed, 96 insertions, 14 deletions

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index f57f88250526..279c5478e8a6 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -151,6 +151,7 @@ extern u64 timecounter_cyc2time(struct timecounter *tc,
  *                      subtraction of non 64 bit counters
  * @mult:               cycle to nanosecond multiplier
  * @shift:              cycle to nanosecond divisor (power of two)
+ * @max_idle_ns:        max idle time permitted by the clocksource (nsecs)
  * @flags:              flags describing special properties
  * @vread:              vsyscall based read
  * @resume:             resume function for the clocksource, if necessary
@@ -168,6 +169,7 @@ struct clocksource {
         cycle_t mask;
         u32 mult;
         u32 shift;
+        u64 max_idle_ns;
         unsigned long flags;
         cycle_t (*vread)(void);
         void (*resume)(void);
diff --git a/include/linux/time.h b/include/linux/time.h
index fe04e5ef6a59..6e026e45a179 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -148,6 +148,7 @@ extern void monotonic_to_bootbased(struct timespec *ts);
 
 extern struct timespec timespec_trunc(struct timespec t, unsigned gran);
 extern int timekeeping_valid_for_hres(void);
+extern u64 timekeeping_max_deferment(void);
 extern void update_wall_time(void);
 extern void update_xtime_cache(u64 nsec);
 extern void timekeeping_leap_insert(int leapsecond);
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 407c0894ef37..b65b242f04dd 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -469,6 +469,47 @@ void clocksource_touch_watchdog(void)
 #ifdef CONFIG_GENERIC_TIME
 
 /**
+ * clocksource_max_deferment - Returns max time the clocksource can be deferred
+ * @cs:         Pointer to clocksource
+ *
+ */
+static u64 clocksource_max_deferment(struct clocksource *cs)
+{
+        u64 max_nsecs, max_cycles;
+
+        /*
+         * Calculate the maximum number of cycles that we can pass to the
+         * cyc2ns function without overflowing a 64-bit signed result. The
+         * maximum number of cycles is equal to ULLONG_MAX/cs->mult which
+         * is equivalent to the below.
+         * max_cycles < (2^63)/cs->mult
+         * max_cycles < 2^(log2((2^63)/cs->mult))
+         * max_cycles < 2^(log2(2^63) - log2(cs->mult))
+         * max_cycles < 2^(63 - log2(cs->mult))
+         * max_cycles < 1 << (63 - log2(cs->mult))
+         * Please note that we add 1 to the result of the log2 to account for
+         * any rounding errors, ensure the above inequality is satisfied and
+         * no overflow will occur.
+         */
+        max_cycles = 1ULL << (63 - (ilog2(cs->mult) + 1));
+
+        /*
+         * The actual maximum number of cycles we can defer the clocksource is
+         * determined by the minimum of max_cycles and cs->mask.
+         */
+        max_cycles = min_t(u64, max_cycles, (u64) cs->mask);
+        max_nsecs = clocksource_cyc2ns(max_cycles, cs->mult, cs->shift);
+
+        /*
+         * To ensure that the clocksource does not wrap whilst we are idle,
+         * limit the time the clocksource can be deferred by 12.5%. Please
+         * note a margin of 12.5% is used because this can be computed with
+         * a shift, versus say 10% which would require division.
+         */
+        return max_nsecs - (max_nsecs >> 5);
+}
+
+/**
  * clocksource_select - Select the best clocksource available
  *
  * Private function. Must hold clocksource_mutex when called.
@@ -564,6 +605,9 @@ static void clocksource_enqueue(struct clocksource *cs)
  */
 int clocksource_register(struct clocksource *cs)
 {
+        /* calculate max idle time permitted for this clocksource */
+        cs->max_idle_ns = clocksource_max_deferment(cs);
+
         mutex_lock(&clocksource_mutex);
         clocksource_enqueue(cs);
         clocksource_select();
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index c65ba0faa98f..a80b4644fe6b 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -208,6 +208,7 @@ void tick_nohz_stop_sched_tick(int inidle)
         struct tick_sched *ts;
         ktime_t last_update, expires, now;
         struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
+        u64 time_delta;
         int cpu;
 
         local_irq_save(flags);
@@ -262,6 +263,17 @@ void tick_nohz_stop_sched_tick(int inidle)
                 seq = read_seqbegin(&xtime_lock);
                 last_update = last_jiffies_update;
                 last_jiffies = jiffies;
+
+                /*
+                 * On SMP we really should only care for the CPU which
+                 * has the do_timer duty assigned. All other CPUs can
+                 * sleep as long as they want.
+                 */
+                if (cpu == tick_do_timer_cpu ||
+                    tick_do_timer_cpu == TICK_DO_TIMER_NONE)
+                        time_delta = timekeeping_max_deferment();
+                else
+                        time_delta = KTIME_MAX;
         } while (read_seqretry(&xtime_lock, seq));
 
         if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
@@ -284,11 +296,26 @@ void tick_nohz_stop_sched_tick(int inidle)
         if ((long)delta_jiffies >= 1) {
 
                 /*
-                * calculate the expiry time for the next timer wheel
-                * timer
-                */
-                expires = ktime_add_ns(last_update, tick_period.tv64 *
-                                   delta_jiffies);
+                 * calculate the expiry time for the next timer wheel
+                 * timer. delta_jiffies >= NEXT_TIMER_MAX_DELTA signals
+                 * that there is no timer pending or at least extremely
+                 * far into the future (12 days for HZ=1000). In this
+                 * case we set the expiry to the end of time.
+                 */
+                if (likely(delta_jiffies < NEXT_TIMER_MAX_DELTA)) {
+                        /*
+                         * Calculate the time delta for the next timer event.
+                         * If the time delta exceeds the maximum time delta
+                         * permitted by the current clocksource then adjust
+                         * the time delta accordingly to ensure the
+                         * clocksource does not wrap.
+                         */
+                        time_delta = min_t(u64, time_delta,
+                                           tick_period.tv64 * delta_jiffies);
+                        expires = ktime_add_ns(last_update, time_delta);
+                } else {
+                        expires.tv64 = KTIME_MAX;
+                }
 
                 /*
                  * If this cpu is the one which updates jiffies, then
@@ -332,22 +359,19 @@ void tick_nohz_stop_sched_tick(int inidle)
 
                 ts->idle_sleeps++;
 
+                /* Mark expires */
+                ts->idle_expires = expires;
+
                 /*
-                 * delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
-                 * there is no timer pending or at least extremly far
-                 * into the future (12 days for HZ=1000). In this case
-                 * we simply stop the tick timer:
+                 * If the expiration time == KTIME_MAX, then
+                 * in this case we simply stop the tick timer.
                  */
-                if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
-                        ts->idle_expires.tv64 = KTIME_MAX;
+                 if (unlikely(expires.tv64 == KTIME_MAX)) {
                         if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
                                 hrtimer_cancel(&ts->sched_timer);
                         goto out;
                 }
 
-                /* Mark expiries */
-                ts->idle_expires = expires;
-
                 if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
                         hrtimer_start(&ts->sched_timer, expires,
                                       HRTIMER_MODE_ABS_PINNED);
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 96b3f0dfa5dc..5d4d4239a0aa 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -478,6 +478,17 @@ int timekeeping_valid_for_hres(void)
 }
 
 /**
+ * timekeeping_max_deferment - Returns max time the clocksource can be deferred
+ *
+ * Caller must observe xtime_lock via read_seqbegin/read_seqretry to
+ * ensure that the clocksource does not change!
+ */
+u64 timekeeping_max_deferment(void)
+{
+        return timekeeper.clock->max_idle_ns;
+}
+
+/**
  * read_persistent_clock -  Return time from the persistent clock.
  *
  * Weak dummy function for arches that do not yet support it.