During sched_domain creation, we define a pack buddy CPU if available.
On a system that share the powerline at all level, the buddy is set to -1
On a dual clusters / dual cores system which can powergate each core and
cluster independantly, the buddy configuration will be :
| CPU0 | CPU1 | CPU2 | CPU3 |
-----------------------------------
buddy | CPU0 | CPU0 | CPU0 | CPU2 |
Small tasks tend to slip out of the periodic load balance.
The best place to choose to migrate them is at their wake up.
Signed-off-by: Vincent Guittot <vincent.guit...@linaro.org>
---
kernel/sched/core.c | 1 +
kernel/sched/fair.c | 109 ++++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched/sched.h | 1 +
3 files changed, 111 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index dab7908..70cadbe 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6131,6 +6131,7 @@ cpu_attach_domain(struct sched_domain *sd, struct
root_domain *rd, int cpu)
rcu_assign_pointer(rq->sd, sd);
destroy_sched_domains(tmp, cpu);
+ update_packing_domain(cpu);
update_top_cache_domain(cpu);
}
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4f4a4f6..8c9d3ed 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -157,6 +157,63 @@ void sched_init_granularity(void)
update_sysctl();
}
+
+/*
+ * Save the id of the optimal CPU that should be used to pack small tasks
+ * The value -1 is used when no buddy has been found
+ */
+DEFINE_PER_CPU(int, sd_pack_buddy);
+
+/* Look for the best buddy CPU that can be used to pack small tasks
+ * We make the assumption that it doesn't wort to pack on CPU that share the
+ * same powerline. We looks for the 1st sched_domain without the
+ * SD_SHARE_POWERLINE flag. Then We look for the sched_group witht the lowest
+ * power per core based on the assumption that their power efficiency is
+ * better */
+void update_packing_domain(int cpu)
+{
+ struct sched_domain *sd;
+ int id = -1;
+
+ sd = highest_flag_domain(cpu, SD_SHARE_POWERLINE);
+ if (!sd)
+ sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd);
+ else
+ sd = sd->parent;
+
+ while (sd) {
+ struct sched_group *sg = sd->groups;
+ struct sched_group *pack = sg;
+ struct sched_group *tmp = sg->next;
+
+ /* 1st CPU of the sched domain is a good candidate */
+ if (id == -1)
+ id = cpumask_first(sched_domain_span(sd));
+
+ /* loop the sched groups to find the best one */
+ while (tmp != sg) {
+ if (tmp->sgp->power * sg->group_weight <
+ sg->sgp->power * tmp->group_weight)
+ pack = tmp;
+ tmp = tmp->next;
+ }
+
+ /* we have found a better group */
+ if (pack != sg)
+ id = cpumask_first(sched_group_cpus(pack));
+
+ /* Look for another CPU than itself */
+ if ((id != cpu)
+ || ((sd->parent) && !(sd->parent->flags && SD_LOAD_BALANCE)))
+ break;
+
+ sd = sd->parent;
+ }
+
+ pr_info(KERN_INFO "CPU%d packing on CPU%d\n", cpu, id);
+ per_cpu(sd_pack_buddy, cpu) = id;
+}
+
#if BITS_PER_LONG == 32
# define WMULT_CONST (~0UL)
#else
@@ -3073,6 +3130,55 @@ static int select_idle_sibling(struct task_struct *p,
int target)
return target;
}
+static inline bool is_buddy_busy(int cpu)
+{
+ struct rq *rq = cpu_rq(cpu);
+
+ /*
+ * A busy buddy is a CPU with a high load or a small load with a lot of
+ * running tasks.
+ */
+ return ((rq->avg.usage_avg_sum << rq->nr_running) >
+ rq->avg.runnable_avg_period);
+}
+
+static inline bool is_light_task(struct task_struct *p)
+{
+ /* A light task runs less than 25% in average */
+ return ((p->se.avg.usage_avg_sum << 2) < p->se.avg.runnable_avg_period);
+}
+
+static int check_pack_buddy(int cpu, struct task_struct *p)
+{
+ int buddy = per_cpu(sd_pack_buddy, cpu);
+
+ /* No pack buddy for this CPU */
+ if (buddy == -1)
+ return false;
+
+ /*
+ * If a task is waiting for running on the CPU which is its own buddy,
+ * let the default behavior to look for a better CPU if available
+ * The threshold has been set to 37.5%
+ */
+ if ((buddy == cpu)
+ && ((p->se.avg.usage_avg_sum << 3) < (p->se.avg.runnable_avg_sum * 5)))
+ return false;
+
+ /* buddy is not an allowed CPU */
+ if (!cpumask_test_cpu(buddy, tsk_cpus_allowed(p)))
+ return false;
+
+ /*
+ * If the task is a small one and the buddy is not overloaded,
+ * we use buddy cpu
+ */
+ if (!is_light_task(p) || is_buddy_busy(buddy))
+ return false;
+
+ return true;
+}
+
/*
* sched_balance_self: balance the current task (running on cpu) in domains
* that have the 'flag' flag set. In practice, this is SD_BALANCE_FORK and
@@ -3098,6 +3204,9 @@ select_task_rq_fair(struct task_struct *p, int sd_flag,
int wake_flags)
if (p->nr_cpus_allowed == 1)
return prev_cpu;
+ if (check_pack_buddy(cpu, p))
+ return per_cpu(sd_pack_buddy, cpu);
+
if (sd_flag & SD_BALANCE_WAKE) {
if (cpumask_test_cpu(cpu, tsk_cpus_allowed(p)))
want_affine = 1;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a95d5c1..086d8bf 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -875,6 +875,7 @@ static inline void idle_balance(int cpu, struct rq *rq)
extern void sysrq_sched_debug_show(void);
extern void sched_init_granularity(void);
+extern void update_packing_domain(int cpu);
extern void update_max_interval(void);
extern void update_group_power(struct sched_domain *sd, int cpu);
extern int update_runtime(struct notifier_block *nfb, unsigned long action,
void *hcpu);
--
1.7.9.5
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