linux-kernel - Re: [patch v7 02/21] sched: set initial value of runnable avg for new forked task

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Date:	Mon, 06 May 2013 08:54:28 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Alex Shi <alex.shi@...el.com>
CC:	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,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, viresh.kumar@...aro.org,
	linux-kernel@...r.kernel.org, len.brown@...el.com,
	rafael.j.wysocki@...el.com, jkosina@...e.cz,
	clark.williams@...il.com, tony.luck@...el.com,
	keescook@...omium.org, mgorman@...e.de, riel@...hat.com
Subject: Re: [patch v7 02/21] sched: set initial value of runnable avg for
 new forked task

Hi Alex,

The below patch looks good to me.

On 04/04/2013 07:30 AM, Alex Shi wrote:
> 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 | 6 ++++++
>  kernel/sched/fair.c | 4 ++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 54eaaa2..8843cd3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1564,6 +1564,7 @@ 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;
>  #endif
>  #ifdef CONFIG_SCHEDSTATS
>  	memset(&p->se.statistics, 0, sizeof(p->se.statistics));
> @@ -1651,6 +1652,11 @@ void sched_fork(struct task_struct *p)
>  		p->sched_reset_on_fork = 0;
>  	}
> 
> +	/* New forked task assumed with full utilization */
> +#if defined(CONFIG_SMP)
> +	p->se.avg.load_avg_contrib = p->se.load.weight;
> +#endif
> +
>  	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;
> 

Reviewed-by:Preeti U Murthy

Thanks

Regards
Preeti U Murthy

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