linux-kernel - Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost whenever newidle balance is attempted

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Date:	Thu, 24 Apr 2014 15:44:47 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Jason Low <jason.low2@...com>
CC:	mingo@...nel.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org,
	alex.shi@...aro.org, efault@....de, vincent.guittot@...aro.org,
	morten.rasmussen@....com, aswin@...com, chegu_vinod@...com
Subject: Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost
 whenever newidle balance is attempted

Hi Jason,

On 04/24/2014 07:00 AM, Jason Low wrote:
> Commit e5fc6611 can potentially cause rq->max_idle_balance_cost to not be
> updated, even when load_balance(NEWLY_IDLE) is attempted and the per-sd
> max cost value is updated.
> 
> In this patch, we update the rq->max_idle_balance_cost regardless of
> whether or not a task has been enqueued while browsing the domains.
> 
> Signed-off-by: Jason Low <jason.low2@...com>
> ---
>  kernel/sched/fair.c |    9 +++++----
>  1 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 43232b8..3e3ffb8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6658,6 +6658,7 @@ static int idle_balance(struct rq *this_rq)
>  	int this_cpu = this_rq->cpu;
> 
>  	idle_enter_fair(this_rq);
> +
>  	/*
>  	 * We must set idle_stamp _before_ calling idle_balance(), such that we
>  	 * measure the duration of idle_balance() as idle time.
> @@ -6710,9 +6711,12 @@ static int idle_balance(struct rq *this_rq)
> 
>  	raw_spin_lock(&this_rq->lock);
> 
> +	if (curr_cost > this_rq->max_idle_balance_cost)
> +		this_rq->max_idle_balance_cost = curr_cost;
> +
>  	/*

What about the update of next_balance field? See the code snippet below.
This will also be skipped as a consequence of the commit e5fc6611 right?

if (pulled_task || time_after(jiffies, this_rq->next_balance)) {
               /*
                  * We are going idle. next_balance may be set based on
                  * a busy processor. So reset next_balance.
                  */
                 this_rq->next_balance = next_balance;
         }

Also the comment in the above snippet does not look right to me.
It says "we are going idle" but the condition checks for pulled_task.


Regards
Preeti U Murthy

>  	 * While browsing the domains, we released the rq lock.
> -	 * A task could have be enqueued in the meantime
> +	 * A task could have been enqueued in the meantime.
>  	 */
>  	if (this_rq->cfs.h_nr_running && !pulled_task) {
>  		pulled_task = 1;
> @@ -6727,9 +6731,6 @@ static int idle_balance(struct rq *this_rq)
>  		this_rq->next_balance = next_balance;
>  	}
> 
> -	if (curr_cost > this_rq->max_idle_balance_cost)
> -		this_rq->max_idle_balance_cost = curr_cost;
> -
>  out:
>  	/* Is there a task of a high priority class? */
>  	if (this_rq->nr_running != this_rq->cfs.h_nr_running)
> 

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