[PATCH] sched/nohz: Optimize get nohz timer target

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

On a machine, cpu 0 is used for housekeeping, other 39 cpus 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.

Cc: Ingo Molnar <mingo@redhat.com> 
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 kernel/sched/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 83bd6bb..db550cf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -548,11 +548,12 @@ int get_nohz_timer_target(void)
 
 	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;
 			}
-- 
2.7.4