[PATCH 4/4] sched: bias to target cpu load to reduce task moving

From: Alex Shi
Date: Sat Nov 23 2013 - 10:18:09 EST


Task migration happens when target just a bit less then source cpu load.
To reduce such situation happens, aggravate the target cpu load with
sd->imbalance_pct/100 in wake_affine.

In find_idlest/busiest_group, change the aggravate to local cpu only
from old group aggravation.

on my pandaboard ES.

latest kernel 527d1511310a89 + whole patchset
hackbench -T -g 10 -f 40
23.25" 21.99"
23.16" 21.20"
24.24" 21.89"
hackbench -p -g 10 -f 40
26.52" 21.46"
23.89" 22.96"
25.65" 22.73"
hackbench -P -g 10 -f 40
20.14" 19.72"
19.96" 19.10"
21.76" 20.03"

Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxx>
---
kernel/sched/fair.c | 35 ++++++++++++++++-------------------
1 file changed, 16 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bccdd89..3623ba4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -978,7 +978,7 @@ static inline unsigned long group_weight(struct task_struct *p, int nid)

static unsigned long weighted_cpuload(const int cpu);
static unsigned long source_load(int cpu);
-static unsigned long target_load(int cpu);
+static unsigned long target_load(int cpu, int imbalance_pct);
static unsigned long power_of(int cpu);
static long effective_load(struct task_group *tg, int cpu, long wl, long wg);

@@ -3809,11 +3809,17 @@ static unsigned long source_load(int cpu)
* Return a high guess at the load of a migration-target cpu weighted
* according to the scheduling class and "nice" value.
*/
-static unsigned long target_load(int cpu)
+static unsigned long target_load(int cpu, int imbalance_pct)
{
struct rq *rq = cpu_rq(cpu);
unsigned long total = weighted_cpuload(cpu);

+ /*
+ * without cpu_load decay, in most of time cpu_load is same as total
+ * so we need to make target a bit heavier to reduce task migration
+ */
+ total = total * imbalance_pct / 100;
+
if (!sched_feat(LB_BIAS))
return total;

@@ -4033,7 +4039,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
this_cpu = smp_processor_id();
prev_cpu = task_cpu(p);
load = source_load(prev_cpu);
- this_load = target_load(this_cpu);
+ this_load = target_load(this_cpu, 100);

/*
* If sync wakeup then subtract the (maximum possible)
@@ -4089,7 +4095,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)

if (balanced ||
(this_load <= load &&
- this_load + target_load(prev_cpu) <= tl_per_task)) {
+ this_load + target_load(prev_cpu, 100) <= tl_per_task)) {
/*
* This domain has SD_WAKE_AFFINE and
* p is cache cold in this domain, and
@@ -4112,7 +4118,6 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
{
struct sched_group *idlest = NULL, *group = sd->groups;
unsigned long min_load = ULONG_MAX, this_load = 0;
- int imbalance = 100 + (sd->imbalance_pct-100)/2;

do {
unsigned long load, avg_load;
@@ -4132,10 +4137,10 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)

for_each_cpu(i, sched_group_cpus(group)) {
/* Bias balancing toward cpus of our domain */
- if (local_group)
+ if (i == this_cpu)
load = source_load(i);
else
- load = target_load(i);
+ load = target_load(i, sd->imbalance_pct);

avg_load += load;
}
@@ -4151,7 +4156,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
}
} while (group = group->next, group != sd->groups);

- if (!idlest || 100*this_load < imbalance*min_load)
+ if (!idlest || this_load < min_load)
return NULL;
return idlest;
}
@@ -5476,9 +5481,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,

nr_running = rq->nr_running;

- /* Bias balancing toward cpus of our domain */
- if (local_group)
- load = target_load(i);
+ /* Bias balancing toward dst cpu */
+ if (env->dst_cpu == i)
+ load = target_load(i, env->sd->imbalance_pct);
else
load = source_load(i);

@@ -5918,14 +5923,6 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
if ((local->idle_cpus < busiest->idle_cpus) &&
busiest->sum_nr_running <= busiest->group_weight)
goto out_balanced;
- } else {
- /*
- * In the CPU_NEWLY_IDLE, CPU_NOT_IDLE cases, use
- * imbalance_pct to be conservative.
- */
- if (100 * busiest->avg_load <=
- env->sd->imbalance_pct * local->avg_load)
- goto out_balanced;
}

force_balance:
--
1.8.1.2

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