linux-kernel - Re: weakness of runnable load tracking?

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Message-ID: <50C33095.9030702@intel.com>
Date:	Sat, 08 Dec 2012 20:20:37 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Paul Turner <pjt@...gle.com>
CC:	Alex Shi <lkml.alex@...il.com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: weakness of runnable load tracking?

On 12/06/2012 11:10 PM, Alex Shi wrote:
> 
>>> Hi Paul & Ingo:
>>>
>>> In a short word of this issue: burst forking/waking tasks have no time
>>> accumulate the load contribute, their runnable load are taken as zero.
>>> that make select_task_rq do a wrong decision on which group is idlest.
>>
>> So these aren't strictly comparable; bursting and forking tasks have
>> fairly different characteristics here.
> 
> Many thanks for looking into this. :)
>>
>> When we fork a task we intentionally reset the previous history.  This
>> means that a forked task that immediately runs is going to show up as
>> 100% runnable and then converge to it's true value.  This was fairly
>> intentionally chosen so that tasks would "start" fast rather than
>> having to worry about ramp up.
> 
> I am sorry for didn't see the 100% runnable for a new forked task. 
> I believe the code need the following patch to initialize decay_count, 
> and load_avg_contrib. otherwise they are random value. 
> In enqueue_entity_load_avg() p->se.avg.runnable_avg_sum for new forked 
> task is always zero, either because se.avg.last_runnable_update is set
>  as clock_task due to decay_count <=0, or just do 
> __synchronize_entity_decay not update_entity_load_avg.

Paul:
Would you like to give some comments for the following patches?

> 
> ===========
> From a161000dbece6e95bf3b81e9246d51784589d393 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 05/12] sched: load tracking bug fix
> 
> We need initialize the se.avg.{decay_count, load_avg_contrib} to zero
> after a new task forked.
> Otherwise random values of above variable give a incorrect statistic
> data when do new task enqueue:
>     enqueue_task_fair
>         enqueue_entity
>             enqueue_entity_load_avg
> 
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> ---
>  kernel/sched/core.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5dae0d2..e6533e1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1534,6 +1534,8 @@ 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;
> +	p->se.avg.load_avg_contrib = 0;
>  #endif
>  #ifdef CONFIG_SCHEDSTATS
>  	memset(&p->se.statistics, 0, sizeof(p->se.statistics));
> 


-- 
Thanks
    Alex
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