linux-kernel - Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task

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Date:	Thu, 9 May 2013 02:30:52 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new
 forked task

On Thu, May 9, 2013 at 1:31 AM, Alex Shi <alex.shi@...el.com> wrote:
>
>>
>> Here is the patch according to Paul's opinions.
>> just refer the __update_task_entity_contrib in sched.h looks ugly.
>> comments are appreciated!
>
> Paul,
>
> With sched_slice, we need to set the runnable avg sum/period after new
> task assigned to a specific CPU.
> So, set them __sched_fork is meaningless. and then

This is still a reasonable choice.

Assuming the system is well balanced, sched_slice() on the current CPU
should be reasonably indicative of the slice wherever we end up.  The
alternative is still to pick a constant.  We should do one of these.

> __update_task_entity_contrib(&p->se) also no reason to use.

Surely we'd still want it so that the right load is added by
enqueue_entity_load_avg()?

> I am going
> to pick up the old patch and drop this one. that also avoid to declare
> it in sched.h.
> What's your comment of this?
>
> Regards!
>>
>> ---
>> From 647404447c996507b6a94110ed13fd122e4ee154 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 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@...el.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)
>>
>
>
> --
> Thanks
>     Alex
--
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