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SubjectRe: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task
On 05/07/2013 11:24 AM, Alex Shi wrote:
>> > The reason to store a small initial "observation" here is so that as
>> > when we reach our next period edge our load converges (presumably
>> > down) towards its true target more smoothly; as well as providing a
>> > task additional protection from being considered "small" through
>> > start-up.
>> >
> It will give new forked task 1 ms extra running time. That will bring
> incorrect info if the new forked goes to sleep a while.
> But this info should benefit to some benchmarks like aim7,
> pthread_cond_broadcast. So I am convinced. :)
>
> What's your opinion of this, Peter?


Here is the patch according to Paul's opinions.
just refer the __update_task_entity_contrib in sched.h looks ugly.
comments are appreciated!

---
From 647404447c996507b6a94110ed13fd122e4ee154 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@intel.com>
Date: Mon, 3 Dec 2012 17:30:39 +0800
Subject: [PATCH 3/7] sched: set initial value of runnable avg for new forked
task

We need initialize the se.avg.{decay_count, load_avg_contrib} for a
new forked task.
Otherwise random values of above variables cause mess when do new task
enqueue:
enqueue_task_fair
enqueue_entity
enqueue_entity_load_avg

and make forking balancing imbalance since incorrect load_avg_contrib.

set avg.decay_count = 0, and give initial value of runnable_avg_sum/period
to resolve such issues.

Thanks for Pual's suggestions

Signed-off-by: Alex Shi <alex.shi@intel.com>
---
kernel/sched/core.c | 8 +++++++-
kernel/sched/fair.c | 4 ++++
kernel/sched/sched.h | 1 +
3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c8db984..4e78de1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1566,6 +1566,11 @@ static void __sched_fork(struct task_struct *p)
#ifdef CONFIG_SMP
p->se.avg.runnable_avg_period = 0;
p->se.avg.runnable_avg_sum = 0;
+ p->se.avg.decay_count = 0;
+ /* New forked task assumed with full utilization */
+ p->se.avg.runnable_avg_period = 1024;
+ p->se.avg.runnable_avg_sum = 1024;
+ __update_task_entity_contrib(&p->se);
#endif
#ifdef CONFIG_SCHEDSTATS
memset(&p->se.statistics, 0, sizeof(p->se.statistics));
@@ -1619,7 +1624,6 @@ void sched_fork(struct task_struct *p)
unsigned long flags;
int cpu = get_cpu();

- __sched_fork(p);
/*
* We mark the process as running here. This guarantees that
* nobody will actually run it, and a signal or other external
@@ -1653,6 +1657,8 @@ void sched_fork(struct task_struct *p)
p->sched_reset_on_fork = 0;
}

+ __sched_fork(p);
+
if (!rt_prio(p->prio))
p->sched_class = &fair_sched_class;

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9c2f726..2881d42 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1508,6 +1508,10 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
* We track migrations using entity decay_count <= 0, on a wake-up
* migration we use a negative decay count to track the remote decays
* accumulated while sleeping.
+ *
+ * When enqueue a new forked task, the se->avg.decay_count == 0, so
+ * we bypass update_entity_load_avg(), use avg.load_avg_contrib initial
+ * value: se->load.weight.
*/
if (unlikely(se->avg.decay_count <= 0)) {
se->avg.last_runnable_update = rq_of(cfs_rq)->clock_task;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c6634f1..ec4cb9b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -876,6 +876,7 @@ extern const struct sched_class idle_sched_class;
extern void trigger_load_balance(struct rq *rq, int cpu);
extern void idle_balance(int this_cpu, struct rq *this_rq);

+extern inline void __update_task_entity_contrib(struct sched_entity *se);
#else /* CONFIG_SMP */

static inline void idle_balance(int cpu, struct rq *rq)
--
1.7.12
--
Thanks
Alex


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