From: Wanpeng Li <wanpen...@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 <mi...@redhat.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Wanpeng Li <wanpen...@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
