nohz: Track last do_timer() cpu

The previous patch which limits the sleep time to the maximum
deferment time of the time keeping clocksource has some limitations on
SMP machines: if all CPUs are idle then for all CPUs the maximum sleep
time is limited.

Solve this by keeping track of which cpu had the do_timer() duty
assigned last and limit the sleep time only for this cpu.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <new-submission>
Cc: Jon Hunter <jon-hunter@ti.com>
Cc: John Stultz <johnstul@us.ibm.com>

2 files changed, 30 insertions, 24 deletions

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 8dc082194b22..d2ae79e21be3 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -43,6 +43,7 @@ enum tick_nohz_mode {
  * @idle_exittime:      Time when the idle state was left
  * @idle_sleeptime:     Sum of the time slept in idle with sched tick stopped
  * @sleep_length:       Duration of the current idle sleep
+ * @do_timer_lst:       CPU was the last one doing do_timer before going idle
  */
 struct tick_sched {
         struct hrtimer                  sched_timer;
@@ -64,6 +65,7 @@ struct tick_sched {
         unsigned long                   last_jiffies;
         unsigned long                   next_jiffies;
         ktime_t                         idle_expires;
+        int                             do_timer_last;
 };
 
 extern void __init tick_init(void);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a80b4644fe6b..df133bc29f89 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -263,17 +263,7 @@ 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;
+                time_delta = timekeeping_max_deferment();
         } while (read_seqretry(&xtime_lock, seq));
 
         if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
@@ -296,6 +286,29 @@ void tick_nohz_stop_sched_tick(int inidle)
         if ((long)delta_jiffies >= 1) {
 
                 /*
+                 * If this cpu is the one which updates jiffies, then
+                 * give up the assignment and let it be taken by the
+                 * cpu which runs the tick timer next, which might be
+                 * this cpu as well. If we don't drop this here the
+                 * jiffies might be stale and do_timer() never
+                 * invoked. Keep track of the fact that it was the one
+                 * which had the do_timer() duty last. If this cpu is
+                 * the one which had the do_timer() duty last, we
+                 * limit the sleep time to the timekeeping
+                 * max_deferement value which we retrieved
+                 * above. Otherwise we can sleep as long as we want.
+                 */
+                if (cpu == tick_do_timer_cpu) {
+                        tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+                        ts->do_timer_last = 1;
+                } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
+                        time_delta = KTIME_MAX;
+                        ts->do_timer_last = 0;
+                } else if (!ts->do_timer_last) {
+                        time_delta = KTIME_MAX;
+                }
+
+                /*
                  * 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
@@ -312,21 +325,12 @@ void tick_nohz_stop_sched_tick(int inidle)
                          */
                         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
-                 * give up the assignment and let it be taken by the
-                 * cpu which runs the tick timer next, which might be
-                 * this cpu as well. If we don't drop this here the
-                 * jiffies might be stale and do_timer() never
-                 * invoked.
-                 */
-                if (cpu == tick_do_timer_cpu)
-                        tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+                if (time_delta < KTIME_MAX)
+                        expires = ktime_add_ns(last_update, time_delta);
+                else
+                        expires.tv64 = KTIME_MAX;
 
                 if (delta_jiffies > 1)
                         cpumask_set_cpu(cpu, nohz_cpu_mask);