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Subject[PATCH v3 05/22] sched: remove domain iterations in fork/exec/wake
Date
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.

This patch increases 10+% performance of hackbench on my 4 sockets
NHM and SNB machines.

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

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ecfbf8e..895a3f4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3364,15 +3364,9 @@ 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;
@@ -3382,18 +3376,6 @@ 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();
--
1.7.12


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