sched: Save some hrtick_start_fair cycles

hrtick_start_fair() shows up in profiles even when disabled.

v3.0.6

taskset -c 3 pipe-test

   PerfTop:     997 irqs/sec  kernel:89.5%  exact:  0.0% [1000Hz cycles],  (all, CPU: 3)
------------------------------------------------------------------------------------------------

             Virgin                                    Patched
             samples  pcnt function                    samples  pcnt function
             _______ _____ ___________________________ _______ _____ ___________________________

             2880.00 10.2% __schedule                  3136.00 11.3% __schedule
             1634.00  5.8% pipe_read                   1615.00  5.8% pipe_read
             1458.00  5.2% system_call                 1534.00  5.5% system_call
             1382.00  4.9% _raw_spin_lock_irqsave      1412.00  5.1% _raw_spin_lock_irqsave
             1202.00  4.3% pipe_write                  1255.00  4.5% copy_user_generic_string
             1164.00  4.1% copy_user_generic_string    1241.00  4.5% __switch_to
             1097.00  3.9% __switch_to                  929.00  3.3% mutex_lock
              872.00  3.1% mutex_lock                   846.00  3.0% mutex_unlock
              687.00  2.4% mutex_unlock                 804.00  2.9% pipe_write
              682.00  2.4% native_sched_clock           713.00  2.6% native_sched_clock
              643.00  2.3% system_call_after_swapgs     653.00  2.3% _raw_spin_unlock_irqrestore
              617.00  2.2% sched_clock_local            633.00  2.3% fsnotify
              612.00  2.2% fsnotify                     605.00  2.2% sched_clock_local
              596.00  2.1% _raw_spin_unlock_irqrestore  593.00  2.1% system_call_after_swapgs
              542.00  1.9% sysret_check                 559.00  2.0% sysret_check
              467.00  1.7% fget_light                   472.00  1.7% fget_light
              462.00  1.6% finish_task_switch           461.00  1.7% finish_task_switch
              437.00  1.5% vfs_write                    442.00  1.6% vfs_write
              431.00  1.5% do_sync_write                428.00  1.5% do_sync_write
              413.00  1.5% select_task_rq_fair          404.00  1.5% _raw_spin_lock_irq
              386.00  1.4% update_curr                  402.00  1.4% update_curr
              385.00  1.4% rw_verify_area               389.00  1.4% do_sync_read
              377.00  1.3% _raw_spin_lock_irq           378.00  1.4% vfs_read
              369.00  1.3% do_sync_read                 340.00  1.2% pipe_iov_copy_from_user
              360.00  1.3% vfs_read                     316.00  1.1% __wake_up_sync_key
*             342.00  1.2% hrtick_start_fair            313.00  1.1% __wake_up_common

Signed-off-by: Mike Galbraith <efault@gmx.de>
[ fixed !CONFIG_SCHED_HRTICK borkage ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1321971607.6855.17.camel@marge.simson.net
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched/fair.c

@@ -2137,7 +2137,7 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
 
 	WARN_ON(task_rq(p) != rq);
 
-	if (hrtick_enabled(rq) && cfs_rq->nr_running > 1) {
+	if (cfs_rq->nr_running > 1) {
 		u64 slice = sched_slice(cfs_rq, se);
 		u64 ran = se->sum_exec_runtime - se->prev_sum_exec_runtime;
 		s64 delta = slice - ran;
@@ -2168,7 +2168,7 @@ static void hrtick_update(struct rq *rq)
 {
 	struct task_struct *curr = rq->curr;
 
-	if (curr->sched_class != &fair_sched_class)
+	if (!hrtick_enabled(rq) || curr->sched_class != &fair_sched_class)
 		return;
 
 	if (cfs_rq_of(&curr->se)->nr_running < sched_nr_latency)
@@ -3031,7 +3031,8 @@ static struct task_struct *pick_next_task_fair(struct rq *rq)
 	} while (cfs_rq);
 
 	p = task_of(se);
-	hrtick_start_fair(rq, p);
+	if (hrtick_enabled(rq))
+		hrtick_start_fair(rq, p);
 
 	return p;
 }

kernel/sched/sched.h

@@ -885,6 +885,13 @@ static inline int hrtick_enabled(struct rq *rq)
 
 void hrtick_start(struct rq *rq, u64 delay);
 
+#else
+
+static inline int hrtick_enabled(struct rq *rq)
+{
+	return 0;
+}
+
 #endif /* CONFIG_SCHED_HRTICK */
 
 #ifdef CONFIG_SMP