linux-kernel - Re: [PATCH] sched: Reduce the rate of needless idle load balancing

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Message-ID: <CAGQ1y=5cwk+-Ztv8kXod+QexwEqQ0U6wPdMDBLL66rcF_cukLA@mail.gmail.com>
Date:	Tue, 20 May 2014 13:51:03 -0700
From:	Jason Low <jason.low2@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <len.brown@...el.com>, Russ Anderson <rja@....com>,
	Dimitri Sivanich <sivanich@....com>,
	Hedi Berriche <hedi@....com>, Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Reduce the rate of needless idle load balancing

On Tue, May 20, 2014 at 1:17 PM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9b4c4f3..97132db 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6764,12 +6764,17 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>
>                 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(rq, CPU_IDLE);
> +               /*
> +                * If time for next balance is due,
> +                * do the balance.
> +                */
> +               if (time_after(jiffies + 1, rq->next_balance)) {

Hi Tim,

If we want to do idle load balancing only when it is due for a
balance, shouldn't the above just be "if (time_after(jiffies,
rq->next_balance))"?

> +                       raw_spin_lock_irq(&rq->lock);
> +                       update_rq_clock(rq);
> +                       update_idle_cpu_load(rq);
> +                       raw_spin_unlock_irq(&rq->lock);
> +                       rebalance_domains(rq, CPU_IDLE);
> +               }
>
>                 if (time_after(this_rq->next_balance, rq->next_balance))
>                         this_rq->next_balance = rq->next_balance;
--
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