5 files changed, 234 insertions, 153 deletions

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 5c69e996bd0f..e934339fbbef 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -144,12 +144,8 @@ struct hrtimer_clock_base *lock_hrtimer_base(const struct hrtimer *timer,
 static int hrtimer_get_target(int this_cpu, int pinned)
 {
 #ifdef CONFIG_NO_HZ
-        if (!pinned && get_sysctl_timer_migration() && idle_cpu(this_cpu)) {
-                int preferred_cpu = get_nohz_load_balancer();
-
-                if (preferred_cpu >= 0)
-                        return preferred_cpu;
-        }
+        if (!pinned && get_sysctl_timer_migration() && idle_cpu(this_cpu))
+                return get_nohz_timer_target();
 #endif
         return this_cpu;
 }
diff --git a/kernel/sched.c b/kernel/sched.c
index a757f6b11cbd..132950b33dde 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -460,7 +460,7 @@ struct rq {
         unsigned long last_load_update_tick;
 #ifdef CONFIG_NO_HZ
         u64 nohz_stamp;
-        unsigned char in_nohz_recently;
+        unsigned char nohz_balance_kick;
 #endif
         unsigned int skip_clock_update;
 
@@ -1195,6 +1195,27 @@ static void resched_cpu(int cpu)
 
 #ifdef CONFIG_NO_HZ
 /*
+ * In the semi idle case, use the nearest busy cpu for migrating timers
+ * from an idle cpu.  This is good for power-savings.
+ *
+ * We don't do similar optimization for completely idle system, as
+ * selecting an idle cpu will add more delays to the timers than intended
+ * (as that cpu's timer base may not be uptodate wrt jiffies etc).
+ */
+int get_nohz_timer_target(void)
+{
+        int cpu = smp_processor_id();
+        int i;
+        struct sched_domain *sd;
+
+        for_each_domain(cpu, sd) {
+                for_each_cpu(i, sched_domain_span(sd))
+                        if (!idle_cpu(i))
+                                return i;
+        }
+        return cpu;
+}
+/*
  * When add_timer_on() enqueues a timer into the timer wheel of an
  * idle CPU then this timer might expire before the next timer event
  * which is scheduled to wake up that CPU. In case of a completely
@@ -7791,6 +7812,10 @@ void __init sched_init(void)
                 rq->idle_stamp = 0;
                 rq->avg_idle = 2*sysctl_sched_migration_cost;
                 rq_attach_root(rq, &def_root_domain);
+#ifdef CONFIG_NO_HZ
+                rq->nohz_balance_kick = 0;
+                init_sched_softirq_csd(&per_cpu(remote_sched_softirq_cb, i));
+#endif
 #endif
                 init_rq_hrtick(rq);
                 atomic_set(&rq->nr_iowait, 0);
@@ -7835,8 +7860,11 @@ void __init sched_init(void)
         zalloc_cpumask_var(&nohz_cpu_mask, GFP_NOWAIT);
 #ifdef CONFIG_SMP
 #ifdef CONFIG_NO_HZ
-        zalloc_cpumask_var(&nohz.cpu_mask, GFP_NOWAIT);
-        alloc_cpumask_var(&nohz.ilb_grp_nohz_mask, GFP_NOWAIT);
+        zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT);
+        alloc_cpumask_var(&nohz.grp_idle_mask, GFP_NOWAIT);
+        atomic_set(&nohz.load_balancer, nr_cpu_ids);
+        atomic_set(&nohz.first_pick_cpu, nr_cpu_ids);
+        atomic_set(&nohz.second_pick_cpu, nr_cpu_ids);
 #endif
         /* May be allocated at isolcpus cmdline parse time */
         if (cpu_isolated_map == NULL)
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 22b8b4f2b616..6ee2e0af665b 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -3091,13 +3091,40 @@ out_unlock:
 }
 
 #ifdef CONFIG_NO_HZ
+
+static DEFINE_PER_CPU(struct call_single_data, remote_sched_softirq_cb);
+
+static void trigger_sched_softirq(void *data)
+{
+        raise_softirq_irqoff(SCHED_SOFTIRQ);
+}
+
+static inline void init_sched_softirq_csd(struct call_single_data *csd)
+{
+        csd->func = trigger_sched_softirq;
+        csd->info = NULL;
+        csd->flags = 0;
+        csd->priv = 0;
+}
+
+/*
+ * idle load balancing details
+ * - One of the idle CPUs nominates itself as idle load_balancer, while
+ *   entering idle.
+ * - This idle load balancer CPU will also go into tickless mode when
+ *   it is idle, just like all other idle CPUs
+ * - When one of the busy CPUs notice that there may be an idle rebalancing
+ *   needed, they will kick the idle load balancer, which then does idle
+ *   load balancing for all the idle CPUs.
+ */
 static struct {
         atomic_t load_balancer;
-        cpumask_var_t cpu_mask;
-        cpumask_var_t ilb_grp_nohz_mask;
-} nohz ____cacheline_aligned = {
-        .load_balancer = ATOMIC_INIT(-1),
-};
+        atomic_t first_pick_cpu;
+        atomic_t second_pick_cpu;
+        cpumask_var_t idle_cpus_mask;
+        cpumask_var_t grp_idle_mask;
+        unsigned long next_balance;     /* in jiffy units */
+} nohz ____cacheline_aligned;
 
 int get_nohz_load_balancer(void)
 {
@@ -3151,17 +3178,17 @@ static inline struct sched_domain *lowest_flag_domain(int cpu, int flag)
  */
 static inline int is_semi_idle_group(struct sched_group *ilb_group)
 {
-        cpumask_and(nohz.ilb_grp_nohz_mask, nohz.cpu_mask,
+        cpumask_and(nohz.grp_idle_mask, nohz.idle_cpus_mask,
                                         sched_group_cpus(ilb_group));
 
         /*
          * A sched_group is semi-idle when it has atleast one busy cpu
          * and atleast one idle cpu.
          */
-        if (cpumask_empty(nohz.ilb_grp_nohz_mask))
+        if (cpumask_empty(nohz.grp_idle_mask))
                 return 0;
 
-        if (cpumask_equal(nohz.ilb_grp_nohz_mask, sched_group_cpus(ilb_group)))
+        if (cpumask_equal(nohz.grp_idle_mask, sched_group_cpus(ilb_group)))
                 return 0;
 
         return 1;
@@ -3194,7 +3221,7 @@ static int find_new_ilb(int cpu)
          * Optimize for the case when we have no idle CPUs or only one
          * idle CPU. Don't walk the sched_domain hierarchy in such cases
          */
-        if (cpumask_weight(nohz.cpu_mask) < 2)
+        if (cpumask_weight(nohz.idle_cpus_mask) < 2)
                 goto out_done;
 
         for_each_flag_domain(cpu, sd, SD_POWERSAVINGS_BALANCE) {
@@ -3202,7 +3229,7 @@ static int find_new_ilb(int cpu)
 
                 do {
                         if (is_semi_idle_group(ilb_group))
-                                return cpumask_first(nohz.ilb_grp_nohz_mask);
+                                return cpumask_first(nohz.grp_idle_mask);
 
                         ilb_group = ilb_group->next;
 
@@ -3210,98 +3237,116 @@ static int find_new_ilb(int cpu)
         }
 
 out_done:
-        return cpumask_first(nohz.cpu_mask);
+        return nr_cpu_ids;
 }
 #else /*  (CONFIG_SCHED_MC || CONFIG_SCHED_SMT) */
 static inline int find_new_ilb(int call_cpu)
 {
-        return cpumask_first(nohz.cpu_mask);
+        return nr_cpu_ids;
 }
 #endif
 
 /*
+ * Kick a CPU to do the nohz balancing, if it is time for it. We pick the
+ * nohz_load_balancer CPU (if there is one) otherwise fallback to any idle
+ * CPU (if there is one).
+ */
+static void nohz_balancer_kick(int cpu)
+{
+        int ilb_cpu;
+
+        nohz.next_balance++;
+
+        ilb_cpu = get_nohz_load_balancer();
+
+        if (ilb_cpu >= nr_cpu_ids) {
+                ilb_cpu = cpumask_first(nohz.idle_cpus_mask);
+                if (ilb_cpu >= nr_cpu_ids)
+                        return;
+        }
+
+        if (!cpu_rq(ilb_cpu)->nohz_balance_kick) {
+                struct call_single_data *cp;
+
+                cpu_rq(ilb_cpu)->nohz_balance_kick = 1;
+                cp = &per_cpu(remote_sched_softirq_cb, cpu);
+                __smp_call_function_single(ilb_cpu, cp, 0);
+        }
+        return;
+}
+
+/*
  * This routine will try to nominate the ilb (idle load balancing)
  * owner among the cpus whose ticks are stopped. ilb owner will do the idle
- * load balancing on behalf of all those cpus. If all the cpus in the system
- * go into this tickless mode, then there will be no ilb owner (as there is
- * no need for one) and all the cpus will sleep till the next wakeup event
- * arrives...
- *
- * For the ilb owner, tick is not stopped. And this tick will be used
- * for idle load balancing. ilb owner will still be part of
- * nohz.cpu_mask..
+ * load balancing on behalf of all those cpus.
  *
- * While stopping the tick, this cpu will become the ilb owner if there
- * is no other owner. And will be the owner till that cpu becomes busy
- * or if all cpus in the system stop their ticks at which point
- * there is no need for ilb owner.
+ * When the ilb owner becomes busy, we will not have new ilb owner until some
+ * idle CPU wakes up and goes back to idle or some busy CPU tries to kick
+ * idle load balancing by kicking one of the idle CPUs.
  *
- * When the ilb owner becomes busy, it nominates another owner, during the
- * next busy scheduler_tick()
+ * Ticks are stopped for the ilb owner as well, with busy CPU kicking this
+ * ilb owner CPU in future (when there is a need for idle load balancing on
+ * behalf of all idle CPUs).
  */
-int select_nohz_load_balancer(int stop_tick)
+void select_nohz_load_balancer(int stop_tick)
 {
         int cpu = smp_processor_id();
 
         if (stop_tick) {
-                cpu_rq(cpu)->in_nohz_recently = 1;
-
                 if (!cpu_active(cpu)) {
                         if (atomic_read(&nohz.load_balancer) != cpu)
-                                return 0;
+                                return;
 
                         /*
                          * If we are going offline and still the leader,
                          * give up!
                          */
-                        if (atomic_cmpxchg(&nohz.load_balancer, cpu, -1) != cpu)
+                        if (atomic_cmpxchg(&nohz.load_balancer, cpu,
+                                           nr_cpu_ids) != cpu)
                                 BUG();
 
-                        return 0;
+                        return;
                 }
 
-                cpumask_set_cpu(cpu, nohz.cpu_mask);
+                cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
 
-                /* time for ilb owner also to sleep */
-                if (cpumask_weight(nohz.cpu_mask) == num_active_cpus()) {
-                        if (atomic_read(&nohz.load_balancer) == cpu)
-                                atomic_set(&nohz.load_balancer, -1);
-                        return 0;
-                }
+                if (atomic_read(&nohz.first_pick_cpu) == cpu)
+                        atomic_cmpxchg(&nohz.first_pick_cpu, cpu, nr_cpu_ids);
+                if (atomic_read(&nohz.second_pick_cpu) == cpu)
+                        atomic_cmpxchg(&nohz.second_pick_cpu, cpu, nr_cpu_ids);
 
-                if (atomic_read(&nohz.load_balancer) == -1) {
-                        /* make me the ilb owner */
-                        if (atomic_cmpxchg(&nohz.load_balancer, -1, cpu) == -1)
-                                return 1;
-                } else if (atomic_read(&nohz.load_balancer) == cpu) {
+                if (atomic_read(&nohz.load_balancer) >= nr_cpu_ids) {
                         int new_ilb;
 
-                        if (!(sched_smt_power_savings ||
-                                                sched_mc_power_savings))
-                                return 1;
+                        /* make me the ilb owner */
+                        if (atomic_cmpxchg(&nohz.load_balancer, nr_cpu_ids,
+                                           cpu) != nr_cpu_ids)
+                                return;
+
                         /*
                          * Check to see if there is a more power-efficient
                          * ilb.
                          */
                         new_ilb = find_new_ilb(cpu);
                         if (new_ilb < nr_cpu_ids && new_ilb != cpu) {
-                                atomic_set(&nohz.load_balancer, -1);
+                                atomic_set(&nohz.load_balancer, nr_cpu_ids);
                                 resched_cpu(new_ilb);
-                                return 0;
+                                return;
                         }
-                        return 1;
+                        return;
                 }
         } else {
-                if (!cpumask_test_cpu(cpu, nohz.cpu_mask))
-                        return 0;
+                if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
+                        return;
 
-                cpumask_clear_cpu(cpu, nohz.cpu_mask);
+                cpumask_clear_cpu(cpu, nohz.idle_cpus_mask);
 
                 if (atomic_read(&nohz.load_balancer) == cpu)
-                        if (atomic_cmpxchg(&nohz.load_balancer, cpu, -1) != cpu)
+                        if (atomic_cmpxchg(&nohz.load_balancer, cpu,
+                                           nr_cpu_ids) != cpu)
                                 BUG();
         }
-        return 0;
+        return;
 }
 #endif
 
@@ -3383,11 +3428,101 @@ out:
                 rq->next_balance = next_balance;
 }
 
+#ifdef CONFIG_NO_HZ
 /*
- * run_rebalance_domains is triggered when needed from the scheduler tick.
- * In CONFIG_NO_HZ case, the idle load balance owner will do the
+ * In CONFIG_NO_HZ case, the idle balance kickee will do the
  * rebalancing for all the cpus for whom scheduler ticks are stopped.
  */
+static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle)
+{
+        struct rq *this_rq = cpu_rq(this_cpu);
+        struct rq *rq;
+        int balance_cpu;
+
+        if (idle != CPU_IDLE || !this_rq->nohz_balance_kick)
+                return;
+
+        for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
+                if (balance_cpu == this_cpu)
+                        continue;
+
+                /*
+                 * If this cpu gets work to do, stop the load balancing
+                 * work being done for other cpus. Next load
+                 * balancing owner will pick it up.
+                 */
+                if (need_resched()) {
+                        this_rq->nohz_balance_kick = 0;
+                        break;
+                }
+
+                raw_spin_lock_irq(&this_rq->lock);
+                update_cpu_load(this_rq);
+                raw_spin_unlock_irq(&this_rq->lock);
+
+                rebalance_domains(balance_cpu, CPU_IDLE);
+
+                rq = cpu_rq(balance_cpu);
+                if (time_after(this_rq->next_balance, rq->next_balance))
+                        this_rq->next_balance = rq->next_balance;
+        }
+        nohz.next_balance = this_rq->next_balance;
+        this_rq->nohz_balance_kick = 0;
+}
+
+/*
+ * Current heuristic for kicking the idle load balancer
+ * - first_pick_cpu is the one of the busy CPUs. It will kick
+ *   idle load balancer when it has more than one process active. This
+ *   eliminates the need for idle load balancing altogether when we have
+ *   only one running process in the system (common case).
+ * - If there are more than one busy CPU, idle load balancer may have
+ *   to run for active_load_balance to happen (i.e., two busy CPUs are
+ *   SMT or core siblings and can run better if they move to different
+ *   physical CPUs). So, second_pick_cpu is the second of the busy CPUs
+ *   which will kick idle load balancer as soon as it has any load.
+ */
+static inline int nohz_kick_needed(struct rq *rq, int cpu)
+{
+        unsigned long now = jiffies;
+        int ret;
+        int first_pick_cpu, second_pick_cpu;
+
+        if (time_before(now, nohz.next_balance))
+                return 0;
+
+        if (!rq->nr_running)
+                return 0;
+
+        first_pick_cpu = atomic_read(&nohz.first_pick_cpu);
+        second_pick_cpu = atomic_read(&nohz.second_pick_cpu);
+
+        if (first_pick_cpu < nr_cpu_ids && first_pick_cpu != cpu &&
+            second_pick_cpu < nr_cpu_ids && second_pick_cpu != cpu)
+                return 0;
+
+        ret = atomic_cmpxchg(&nohz.first_pick_cpu, nr_cpu_ids, cpu);
+        if (ret == nr_cpu_ids || ret == cpu) {
+                atomic_cmpxchg(&nohz.second_pick_cpu, cpu, nr_cpu_ids);
+                if (rq->nr_running > 1)
+                        return 1;
+        } else {
+                ret = atomic_cmpxchg(&nohz.second_pick_cpu, nr_cpu_ids, cpu);
+                if (ret == nr_cpu_ids || ret == cpu) {
+                        if (rq->nr_running)
+                                return 1;
+                }
+        }
+        return 0;
+}
+#else
+static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle) { }
+#endif
+
+/*
+ * run_rebalance_domains is triggered when needed from the scheduler tick.
+ * Also triggered for nohz idle balancing (with nohz_balancing_kick set).
+ */
 static void run_rebalance_domains(struct softirq_action *h)
 {
         int this_cpu = smp_processor_id();
@@ -3397,40 +3532,12 @@ static void run_rebalance_domains(struct softirq_action *h)
 
         rebalance_domains(this_cpu, idle);
 
-#ifdef CONFIG_NO_HZ
         /*
-         * If this cpu is the owner for idle load balancing, then do the
+         * If this cpu has a pending nohz_balance_kick, then do the
          * balancing on behalf of the other idle cpus whose ticks are
          * stopped.
          */
-        if (this_rq->idle_at_tick &&
-            atomic_read(&nohz.load_balancer) == this_cpu) {
-                struct rq *rq;
-                int balance_cpu;
-
-                for_each_cpu(balance_cpu, nohz.cpu_mask) {
-                        if (balance_cpu == this_cpu)
-                                continue;
-
-                        /*
-                         * If this cpu gets work to do, stop the load balancing
-                         * work being done for other cpus. Next load
-                         * balancing owner will pick it up.
-                         */
-                        if (need_resched())
-                                break;
-
-                        rq = cpu_rq(balance_cpu);
-                        raw_spin_lock_irq(&rq->lock);
-                        update_cpu_load(rq);
-                        raw_spin_unlock_irq(&rq->lock);
-                        rebalance_domains(balance_cpu, CPU_IDLE);
-
-                        if (time_after(this_rq->next_balance, rq->next_balance))
-                                this_rq->next_balance = rq->next_balance;
-                }
-        }
-#endif
+        nohz_idle_balance(this_cpu, idle);
 }
 
 static inline int on_null_domain(int cpu)
@@ -3440,57 +3547,17 @@ static inline int on_null_domain(int cpu)
 
 /*
  * Trigger the SCHED_SOFTIRQ if it is time to do periodic load balancing.
- *
- * In case of CONFIG_NO_HZ, this is the place where we nominate a new
- * idle load balancing owner or decide to stop the periodic load balancing,
- * if the whole system is idle.
  */
 static inline void trigger_load_balance(struct rq *rq, int cpu)
 {
-#ifdef CONFIG_NO_HZ
-        /*
-         * If we were in the nohz mode recently and busy at the current
-         * scheduler tick, then check if we need to nominate new idle
-         * load balancer.
-         */
-        if (rq->in_nohz_recently && !rq->idle_at_tick) {
-                rq->in_nohz_recently = 0;
-
-                if (atomic_read(&nohz.load_balancer) == cpu) {
-                        cpumask_clear_cpu(cpu, nohz.cpu_mask);
-                        atomic_set(&nohz.load_balancer, -1);
-                }
-
-                if (atomic_read(&nohz.load_balancer) == -1) {
-                        int ilb = find_new_ilb(cpu);
-
-                        if (ilb < nr_cpu_ids)
-                                resched_cpu(ilb);
-                }
-        }
-
-        /*
-         * If this cpu is idle and doing idle load balancing for all the
-         * cpus with ticks stopped, is it time for that to stop?
-         */
-        if (rq->idle_at_tick && atomic_read(&nohz.load_balancer) == cpu &&
-            cpumask_weight(nohz.cpu_mask) == num_online_cpus()) {
-                resched_cpu(cpu);
-                return;
-        }
-
-        /*
-         * If this cpu is idle and the idle load balancing is done by
-         * someone else, then no need raise the SCHED_SOFTIRQ
-         */
-        if (rq->idle_at_tick && atomic_read(&nohz.load_balancer) != cpu &&
-            cpumask_test_cpu(cpu, nohz.cpu_mask))
-                return;
-#endif
         /* Don't need to rebalance while attached to NULL domain */
         if (time_after_eq(jiffies, rq->next_balance) &&
             likely(!on_null_domain(cpu)))
                 raise_softirq(SCHED_SOFTIRQ);
+#ifdef CONFIG_NO_HZ
+        else if (nohz_kick_needed(rq, cpu) && likely(!on_null_domain(cpu)))
+                nohz_balancer_kick(cpu);
+#endif
 }
 
 static void rq_online_fair(struct rq *rq)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 1d7b9bc1c034..5f171f04ab00 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -408,13 +408,7 @@ void tick_nohz_stop_sched_tick(int inidle)
                  * the scheduler tick in nohz_restart_sched_tick.
                  */
                 if (!ts->tick_stopped) {
-                        if (select_nohz_load_balancer(1)) {
-                                /*
-                                 * sched tick not stopped!
-                                 */
-                                cpumask_clear_cpu(cpu, nohz_cpu_mask);
-                                goto out;
-                        }
+                        select_nohz_load_balancer(1);
 
                         ts->idle_tick = hrtimer_get_expires(&ts->sched_timer);
                         ts->tick_stopped = 1;
diff --git a/kernel/timer.c b/kernel/timer.c
index ee305c8d4e18..48d6aec0789c 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -679,12 +679,8 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
         cpu = smp_processor_id();
 
 #if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
-        if (!pinned && get_sysctl_timer_migration() && idle_cpu(cpu)) {
-                int preferred_cpu = get_nohz_load_balancer();
-
-                if (preferred_cpu >= 0)
-                        cpu = preferred_cpu;
-        }
+        if (!pinned && get_sysctl_timer_migration() && idle_cpu(cpu))
+                cpu = get_nohz_timer_target();
 #endif
         new_base = per_cpu(tvec_bases, cpu);