linux-kernel - Re: [patch v5 02/15] sched: set initial load avg of new forked task

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Message-ID: <51246B1A.8040100@intel.com>
Date:	Wed, 20 Feb 2013 14:20:10 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Alex Shi <alex.shi@...el.com>
CC:	torvalds@...ux-foundation.org, mingo@...hat.com,
	peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, arjan@...ux.intel.com, bp@...en8.de,
	pjt@...gle.com, namhyung@...nel.org, efault@....de,
	vincent.guittot@...aro.org, gregkh@...uxfoundation.org,
	preeti@...ux.vnet.ibm.com, viresh.kumar@...aro.org,
	linux-kernel@...r.kernel.org, morten.rasmussen@....com
Subject: Re: [patch v5 02/15] sched: set initial load avg of new forked task

On 02/18/2013 01:07 PM, Alex Shi wrote:
> New task has no runnable sum at its first runnable time, so its
> runnable load is zero. That makes burst forking balancing just select
> few idle cpus to assign tasks if we engage runnable load in balancing.
> 
> Set initial load avg of new forked task as its load weight to resolve
> this issue.
> 

patch answering PJT's update here. that merged the 1st and 2nd patches 
into one. other patches in serial don't need to change.

=========
>From 89b56f2e5a323a0cb91c98be15c94d34e8904098 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@...el.com>
Date: Mon, 3 Dec 2012 17:30:39 +0800
Subject: [PATCH 01/14] 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 avg.load_avg_contrib = se->load.weight to
resolve such issues.

Signed-off-by: Alex Shi <alex.shi@...el.com>
---
 kernel/sched/core.c | 3 +++
 kernel/sched/fair.c | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 26058d0..1452e14 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1559,6 +1559,7 @@ static void __sched_fork(struct task_struct *p)
 #if defined(CONFIG_SMP) && defined(CONFIG_FAIR_GROUP_SCHED)
 	p->se.avg.runnable_avg_period = 0;
 	p->se.avg.runnable_avg_sum = 0;
+	p->se.avg.decay_count = 0;
 #endif
 #ifdef CONFIG_SCHEDSTATS
 	memset(&p->se.statistics, 0, sizeof(p->se.statistics));
@@ -1646,6 +1647,8 @@ void sched_fork(struct task_struct *p)
 		p->sched_reset_on_fork = 0;
 	}
 
+	p->se.avg.load_avg_contrib = p->se.load.weight;
+
 	if (!rt_prio(p->prio))
 		p->sched_class = &fair_sched_class;
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 81fa536..cae5134 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1509,6 +1509,10 @@ 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;
-- 
1.7.12

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