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Subject[PATCH 3/3] sched/fair: fix unnecessary increase of balance interval
Date
In case of active balance, we increase the balance interval to cover
pinned tasks cases not covered by all_pinned logic. Neverthless, the
active migration triggered by asym packing should be treated as the normal
unbalanced case and reset the interval to default value otherwise active
migration for asym_packing can be easily delayed for hundreds of ms
because of this pinned task detection mecanism.
The same happen to other conditions tested in need_active_balance() like
mistfit task and when the capacity of src_cpu is reduced compared to
dst_cpu (see comments in need_active_balance() for details).

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/fair.c | 40 +++++++++++++++++++++++++++-------------
1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 487c73e..9b1e701 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8849,21 +8849,25 @@ static struct rq *find_busiest_queue(struct lb_env *env,
*/
#define MAX_PINNED_INTERVAL 512

-static int need_active_balance(struct lb_env *env)
+static inline bool
+asym_active_balance(struct lb_env *env)
{
- struct sched_domain *sd = env->sd;
+ /*
+ * ASYM_PACKING needs to force migrate tasks from busy but
+ * lower priority CPUs in order to pack all tasks in the
+ * highest priority CPUs.
+ */
+ return env->idle != CPU_NOT_IDLE && (env->sd->flags & SD_ASYM_PACKING) &&
+ sched_asym_prefer(env->dst_cpu, env->src_cpu);
+}

- if (env->idle != CPU_NOT_IDLE) {
+static inline bool
+voluntary_active_balance(struct lb_env *env)
+{
+ struct sched_domain *sd = env->sd;

- /*
- * ASYM_PACKING needs to force migrate tasks from busy but
- * lower priority CPUs in order to pack all tasks in the
- * highest priority CPUs.
- */
- if ((sd->flags & SD_ASYM_PACKING) &&
- sched_asym_prefer(env->dst_cpu, env->src_cpu))
- return 1;
- }
+ if (asym_active_balance(env))
+ return 1;

/*
* The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
@@ -8881,6 +8885,16 @@ static int need_active_balance(struct lb_env *env)
if (env->src_grp_type == group_misfit_task)
return 1;

+ return 0;
+}
+
+static int need_active_balance(struct lb_env *env)
+{
+ struct sched_domain *sd = env->sd;
+
+ if (voluntary_active_balance(env))
+ return 1;
+
return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
}

@@ -9142,7 +9156,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
} else
sd->nr_balance_failed = 0;

- if (likely(!active_balance)) {
+ if (likely(!active_balance) || voluntary_active_balance(&env)) {
/* We were unbalanced, so reset the balancing interval */
sd->balance_interval = sd->min_interval;
} else {
--
2.7.4
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