linux-kernel - Re: [PATCH v3 07/22] sched: set initial load avg of new forked task

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Date:	Fri, 11 Jan 2013 10:40:41 +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,
	vincent.guittot@...aro.org, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 07/22] sched: set initial load avg of new forked task

On 01/05/2013 02:07 PM, Alex Shi wrote:
> New task has no runnable sum at its first runnable time, that make
> burst forking just select few idle cpus to put tasks.
> Set initial load avg of new forked task as its load weight to resolve
> this issue.
> 
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> ---
>  include/linux/sched.h |  1 +
>  kernel/sched/core.c   |  2 +-
>  kernel/sched/fair.c   | 11 +++++++++--
>  3 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 206bb08..fb7aab5 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1069,6 +1069,7 @@ struct sched_domain;
>  #else
>  #define ENQUEUE_WAKING		0
>  #endif
> +#define ENQUEUE_NEWTASK		8
> 
>  #define DEQUEUE_SLEEP		1
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 66c1718..66ce1f1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1705,7 +1705,7 @@ void wake_up_new_task(struct task_struct *p)
>  #endif
> 
>  	rq = __task_rq_lock(p);
> -	activate_task(rq, p, 0);
> +	activate_task(rq, p, ENQUEUE_NEWTASK);
>  	p->on_rq = 1;
>  	trace_sched_wakeup_new(p, true);
>  	check_preempt_curr(rq, p, WF_FORK);
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 895a3f4..5c545e4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1503,8 +1503,9 @@ static inline void update_rq_runnable_avg(struct rq *rq, int runnable)
>  /* Add the load generated by se into cfs_rq's child load-average */
>  static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
>  						  struct sched_entity *se,
> -						  int wakeup)
> +						  int flags)
>  {
> +	int wakeup = flags & ENQUEUE_WAKEUP;
>  	/*
>  	 * We track migrations using entity decay_count <= 0, on a wake-up
>  	 * migration we use a negative decay count to track the remote decays
> @@ -1538,6 +1539,12 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
>  		update_entity_load_avg(se, 0);
>  	}
> 
> +	/*
> +	 * set the initial load avg of new task same as its load
> +	 * in order to avoid brust fork make few cpu too heavier
> +	 */
> +	if (flags & ENQUEUE_NEWTASK)
> +		se->avg.load_avg_contrib = se->load.weight;
>  	cfs_rq->runnable_load_avg += se->avg.load_avg_contrib;
>  	/* we force update consideration on load-balancer moves */
>  	update_cfs_rq_blocked_load(cfs_rq, !wakeup);
> @@ -1701,7 +1708,7 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>  	 * Update run-time statistics of the 'current'.
>  	 */
>  	update_curr(cfs_rq);
> -	enqueue_entity_load_avg(cfs_rq, se, flags & ENQUEUE_WAKEUP);
> +	enqueue_entity_load_avg(cfs_rq, se, flags);
>  	account_entity_enqueue(cfs_rq, se);
>  	update_cfs_shares(cfs_rq);
> 
I had seen in my experiments, that the forked tasks with initial load to
be 0,would adversely affect the runqueue lengths.Since the load for
these tasks to pick up takes some time,the cpus on which the forked
tasks are scheduled, could be candidates for "dst_cpu" many times and
the runqueue lengths increase considerably.

This patch solves this issue by making the forked tasks contribute
actively to the runqueue load.

Reviewed-by:Preeti U Murthy

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