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Subject[PATCH v3 01/10] sched/fair: clean up asym packing
Date
Clean up asym packing to follow the default load balance behavior:
- classify the group by creating a group_asym_packing field.
- calculate the imbalance in calculate_imbalance() instead of bypassing it.

We don't need to test twice same conditions anymore to detect asym packing
and we consolidate the calculation of imbalance in calculate_imbalance().

There is no functional changes.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/fair.c | 63 ++++++++++++++---------------------------------------
1 file changed, 16 insertions(+), 47 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1054d2c..3175fea 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7685,6 +7685,7 @@ struct sg_lb_stats {
unsigned int group_weight;
enum group_type group_type;
int group_no_capacity;
+ unsigned int group_asym_packing; /* Tasks should be moved to preferred CPU */
unsigned long group_misfit_task_load; /* A CPU has a task too big for its capacity */
#ifdef CONFIG_NUMA_BALANCING
unsigned int nr_numa_running;
@@ -8139,9 +8140,17 @@ static bool update_sd_pick_busiest(struct lb_env *env,
* ASYM_PACKING needs to move all the work to the highest
* prority CPUs in the group, therefore mark all groups
* of lower priority than ourself as busy.
+ *
+ * This is primarily intended to used at the sibling level. Some
+ * cores like POWER7 prefer to use lower numbered SMT threads. In the
+ * case of POWER7, it can move to lower SMT modes only when higher
+ * threads are idle. When in lower SMT modes, the threads will
+ * perform better since they share less core resources. Hence when we
+ * have idle threads, we want them to be the higher ones.
*/
if (sgs->sum_nr_running &&
sched_asym_prefer(env->dst_cpu, sg->asym_prefer_cpu)) {
+ sgs->group_asym_packing = 1;
if (!sds->busiest)
return true;

@@ -8283,51 +8292,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
}

/**
- * check_asym_packing - Check to see if the group is packed into the
- * sched domain.
- *
- * This is primarily intended to used at the sibling level. Some
- * cores like POWER7 prefer to use lower numbered SMT threads. In the
- * case of POWER7, it can move to lower SMT modes only when higher
- * threads are idle. When in lower SMT modes, the threads will
- * perform better since they share less core resources. Hence when we
- * have idle threads, we want them to be the higher ones.
- *
- * This packing function is run on idle threads. It checks to see if
- * the busiest CPU in this domain (core in the P7 case) has a higher
- * CPU number than the packing function is being run on. Here we are
- * assuming lower CPU number will be equivalent to lower a SMT thread
- * number.
- *
- * Return: 1 when packing is required and a task should be moved to
- * this CPU. The amount of the imbalance is returned in env->imbalance.
- *
- * @env: The load balancing environment.
- * @sds: Statistics of the sched_domain which is to be packed
- */
-static int check_asym_packing(struct lb_env *env, struct sd_lb_stats *sds)
-{
- int busiest_cpu;
-
- if (!(env->sd->flags & SD_ASYM_PACKING))
- return 0;
-
- if (env->idle == CPU_NOT_IDLE)
- return 0;
-
- if (!sds->busiest)
- return 0;
-
- busiest_cpu = sds->busiest->asym_prefer_cpu;
- if (sched_asym_prefer(busiest_cpu, env->dst_cpu))
- return 0;
-
- env->imbalance = sds->busiest_stat.group_load;
-
- return 1;
-}
-
-/**
* fix_small_imbalance - Calculate the minor imbalance that exists
* amongst the groups of a sched_domain, during
* load balancing.
@@ -8411,6 +8375,11 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
local = &sds->local_stat;
busiest = &sds->busiest_stat;

+ if (busiest->group_asym_packing) {
+ env->imbalance = busiest->group_load;
+ return;
+ }
+
if (busiest->group_type == group_imbalanced) {
/*
* In the group_imb case we cannot rely on group-wide averages
@@ -8515,8 +8484,8 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
busiest = &sds.busiest_stat;

/* ASYM feature bypasses nice load balance check */
- if (check_asym_packing(env, &sds))
- return sds.busiest;
+ if (busiest->group_asym_packing)
+ goto force_balance;

/* There is no busy sibling group to pull tasks from */
if (!sds.busiest || busiest->sum_nr_running == 0)
--
2.7.4
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