Re: [PATCH 3/8] sched: don't consider other cpus in our group in case of NEWLY_IDLE

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2013-02-14 at 14:48 +0900, Joonsoo Kim wrote:
> Commit 88b8dac0 makes load_balance() consider other cpus in its group,
> regardless of idle type. When we do NEWLY_IDLE balancing, we should not
> consider it, because a motivation of NEWLY_IDLE balancing is to turn
> this cpu to non idle state if needed. This is not the case of other cpus.
> So, change code not to consider other cpus for NEWLY_IDLE balancing.
> 
> With this patch, assign 'if (pulled_task) this_rq->idle_stamp = 0'
> in idle_balance() is corrected, because NEWLY_IDLE balancing doesn't
> consider other cpus. Assigning to 'this_rq->idle_stamp' is now valid.
> 
> Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Fair enough, good catch.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0c6aaf6..97498f4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5016,8 +5016,15 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>  		.cpus		= cpus,
>  	};
>  
> +	/* For NEWLY_IDLE load_balancing, we don't need to consider
> +	 * other cpus in our group */
> +	if (idle == CPU_NEWLY_IDLE) {
> +		env.dst_grpmask = NULL;
> +		max_lb_iterations = 0;

Just a small nit; I don't think we'll ever get to evaluate
max_lb_iterations when !dst_grpmask. So strictly speaking its
superfluous to touch it.

> +	} else {
> +		max_lb_iterations = cpumask_weight(env.dst_grpmask);
> +	}
>  	cpumask_copy(cpus, cpu_active_mask);
> -	max_lb_iterations = cpumask_weight(env.dst_grpmask);
>  
>  	schedstat_inc(sd, lb_count[idle]);
>