linux-kernel - Re: [RFC] sched: nohz_idle

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Date:	Thu, 13 Sep 2012 08:49:51 +0200
From:	Mike Galbraith <efault@....de>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	peterz@...radead.org, mingo@...hat.com
Subject: Re: [RFC] sched: nohz_idle_balance

On Thu, 2012-09-13 at 06:11 +0200, Vincent Guittot wrote: 
> On tickless system, one CPU runs load balance for all idle CPUs.
> The cpu_load of this CPU is updated before starting the load balance
> of each other idle CPUs. We should instead update the cpu_load of the balance_cpu.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c |   11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1ca4fe4..9ae3a5b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4794,14 +4794,15 @@ static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle)
>  		if (need_resched())
>  			break;
>  
> -		raw_spin_lock_irq(&this_rq->lock);
> -		update_rq_clock(this_rq);
> -		update_idle_cpu_load(this_rq);
> -		raw_spin_unlock_irq(&this_rq->lock);
> +		rq = cpu_rq(balance_cpu);
> +
> +		raw_spin_lock_irq(&rq->lock);
> +		update_rq_clock(rq);
> +		update_idle_cpu_load(rq);
> +		raw_spin_unlock_irq(&rq->lock);
>  
>  		rebalance_domains(balance_cpu, CPU_IDLE);
>  
> -		rq = cpu_rq(balance_cpu);
>  		if (time_after(this_rq->next_balance, rq->next_balance))
>  			this_rq->next_balance = rq->next_balance;
>  	}

Ew, banging locks and updating clocks to what good end?

-Mike

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