linux-kernel - Re: [PATCH 3/3] sched: add_nr_running(): drop tick_nohz_full

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Message-ID: <20150924144418.GB8569@lerouge>
Date:	Thu, 24 Sep 2015 16:44:18 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Luiz Capitulino <lcapitulino@...hat.com>
Cc:	linux-kernel@...r.kernel.org, cmetcalf@...hip.com
Subject: Re: [PATCH 3/3] sched: add_nr_running(): drop tick_nohz_full_cpu()
 check

On Thu, Sep 10, 2015 at 03:58:27PM -0400, Luiz Capitulino wrote:
> tick_nohz_full_kick_cpu() performs the same check.
> 
> Signed-off-by: Luiz Capitulino <lcapitulino@...hat.com>
> ---
>  kernel/sched/sched.h | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 68cda11..102eb18 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1323,17 +1323,15 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
>  #endif
>  
>  #ifdef CONFIG_NO_HZ_FULL
> -		if (tick_nohz_full_cpu(rq->cpu)) {
> -			/*
> -			 * Tick is needed if more than one task runs on a CPU.
> -			 * Send the target an IPI to kick it out of nohz mode.
> -			 *
> -			 * We assume that IPI implies full memory barrier and the
> -			 * new value of rq->nr_running is visible on reception
> -			 * from the target.
> -			 */
> -			tick_nohz_full_kick_cpu(rq->cpu);
> -		}
> +		/*
> +		 * Tick is needed if more than one task runs on a CPU.
> +		 * Send the target an IPI to kick it out of nohz mode.
> +		 *
> +		 * We assume that IPI implies full memory barrier and the
> +		 * new value of rq->nr_running is visible on reception
> +		 * from the target.
> +		 */
> +		tick_nohz_full_kick_cpu(rq->cpu);

Nope, we want to keep this because tick_nohz_full_cpu() does a static key check.
Most users don't care about nohz_full and I really want to keep nohz full off case
overhead to the bare minimum.

Thanks.

>  #endif
>  	}
>  }
> -- 
> 2.1.0
> 
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