[PATCH v4 2/2] sched/nohz: Optimize get_nohz_timer_target()

[Wanpeng Li]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

On a machine, cpu 0 is used for housekeeping, the other 39 cpus in the 
same socket are in nohz_full mode. We can observe huge time burn in the 
loop for seaching nearest busy housekeeper cpu by ftrace.

  2)               |       get_nohz_timer_target() {
  2)   0.240 us    |         housekeeping_test_cpu();
  2)   0.458 us    |         housekeeping_test_cpu();

  ...

  2)   0.292 us    |         housekeeping_test_cpu();
  2)   0.240 us    |         housekeeping_test_cpu();
  2)   0.227 us    |         housekeeping_any_cpu();
  2) + 43.460 us   |       }
  
This patch optimizes the searching logic by finding a nearest housekeeper
cpu in the housekeeping cpumask, it can minimize the worst searching time 
from ~44us to < 10us in my testing. In addition, the last iterated busy 
housekeeper can become a random candidate while current CPU is a better 
fallback if it is a housekeeper.

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx> 
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
v1 -> v2:
 * current CPU is a better fallback if it is a housekeeper

 kernel/sched/core.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 102dfcf..04a0f6a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -539,27 +539,32 @@ void resched_cpu(int cpu)
  */
 int get_nohz_timer_target(void)
 {
-	int i, cpu = smp_processor_id();
+	int i, cpu = smp_processor_id(), default_cpu = -1;
 	struct sched_domain *sd;
 
-	if (!idle_cpu(cpu) && housekeeping_cpu(cpu, HK_FLAG_TIMER))
-		return cpu;
+	if (housekeeping_cpu(cpu, HK_FLAG_TIMER)) {
+		if (!idle_cpu(cpu))
+			return cpu;
+		default_cpu = cpu;
+	}
 
 	rcu_read_lock();
 	for_each_domain(cpu, sd) {
-		for_each_cpu(i, sched_domain_span(sd)) {
+		for_each_cpu_and(i, sched_domain_span(sd),
+			housekeeping_cpumask(HK_FLAG_TIMER)) {
 			if (cpu == i)
 				continue;
 
-			if (!idle_cpu(i) && housekeeping_cpu(i, HK_FLAG_TIMER)) {
+			if (!idle_cpu(i)) {
 				cpu = i;
 				goto unlock;
 			}
 		}
 	}
 
-	if (!housekeeping_cpu(cpu, HK_FLAG_TIMER))
-		cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
+	if (default_cpu == -1)
+		default_cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
+	cpu = default_cpu;
 unlock:
 	rcu_read_unlock();
 	return cpu;
-- 
1.8.3.1