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>

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 8dc0821..d2ae79e 100644 (file)
--- 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 a80b464..df133bc 100644 (file)
--- 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);