[PATCH 04/10] sched: remove domain iterations in fork/exec/wake

[Alex Shi]

Guess the search cpu from bottom to up in domain tree come from
commit 3dbd5342074a1e sched: multilevel sbe sbf, the purpose is
balancing over tasks on all level domains.

This balancing cost much if there has many domain/groups in a large
system. And force spreading task among different domains may cause
performance issue due to bad locality.

If we remove this code, we will get quick fork/exec/wake, plus better
balancing among whole system, that also reduce migrations in future
load balancing.

Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
---
 kernel/sched/fair.c |   20 +-------------------
 1 files changed, 1 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 05ee54e..1faf89f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3136,15 +3136,9 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
 		goto unlock;
 	}
 
-	while (sd) {
+	if (sd) {
 		int load_idx = sd->forkexec_idx;
 		struct sched_group *group;
-		int weight;
-
-		if (!(sd->flags & sd_flag)) {
-			sd = sd->child;
-			continue;
-		}
 
 		if (sd_flag & SD_BALANCE_WAKE)
 			load_idx = sd->wake_idx;
@@ -3154,18 +3148,6 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
 			goto unlock;
 
 		new_cpu = find_idlest_cpu(group, p, cpu);
-
-		/* Now try balancing at a lower domain level of new_cpu */
-		cpu = new_cpu;
-		weight = sd->span_weight;
-		sd = NULL;
-		for_each_domain(cpu, tmp) {
-			if (weight <= tmp->span_weight)
-				break;
-			if (tmp->flags & sd_flag)
-				sd = tmp;
-		}
-		/* while loop will break here if sd == NULL */
 	}
 unlock:
 	rcu_read_unlock();
-- 
1.7.5.4

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