linux-kernel - Re: [PATCH 1/2] Tick: Introduce tick_nohz

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Date:	Tue, 29 Oct 2013 02:51:35 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lan Tianyu <tianyu.lan@...el.com>
Cc:	rjw@...ysocki.net, viresh.kumar@...aro.org, dipankar@...ibm.com,
	fweisbec@...il.com, youquan.song@...el.com, riel@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Tick: Introduce tick_nohz_check() to check nohz
 enable status

On Tue, Oct 29, 2013 at 04:48:56PM +0800, Lan Tianyu wrote:
> In some cases, nohz enable status needs to be checked. E.G, in RCU and cpufreq
> ondemand governor. So add tick_nohz_check() to return tick_nohz_enabled value
> And use tick_nohz_check() instead of referencing tick_nohz_enabled in the rcutree_plugin.h.
> 
> Signed-off-by: Lan Tianyu <tianyu.lan@...el.com>

NACK on the rcutree change unless you put the ACCESS_ONCE() in.

Or is there some reason that ACCESS_ONCE() is not needed?  If so, what
is that reason?

							Thanx, Paul

> ---
>  include/linux/tick.h     | 2 ++
>  kernel/rcutree_plugin.h  | 4 +---
>  kernel/time/tick-sched.c | 8 +++++++-
>  3 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 5128d33..a9c5374 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -136,6 +136,7 @@ static inline int tick_nohz_tick_stopped(void)
>  extern void tick_nohz_idle_enter(void);
>  extern void tick_nohz_idle_exit(void);
>  extern void tick_nohz_irq_exit(void);
> +extern int tick_nohz_check(void);
>  extern ktime_t tick_nohz_get_sleep_length(void);
>  extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
>  extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
> @@ -155,6 +156,7 @@ static inline ktime_t tick_nohz_get_sleep_length(void)
> 
>  	return len;
>  }
> +static inline int tick_nohz_check(void) { return 0; }
>  static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
>  static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
>  # endif /* !CONFIG_NO_HZ_COMMON */
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 130c97b..af167ec 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -1627,8 +1627,6 @@ module_param(rcu_idle_gp_delay, int, 0644);
>  static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY;
>  module_param(rcu_idle_lazy_gp_delay, int, 0644);
> 
> -extern int tick_nohz_enabled;
> -
>  /*
>   * Try to advance callbacks for all flavors of RCU on the current CPU.
>   * Afterwards, if there are any callbacks ready for immediate invocation,
> @@ -1718,7 +1716,7 @@ static void rcu_prepare_for_idle(int cpu)
>  	int tne;
> 
>  	/* Handle nohz enablement switches conservatively. */
> -	tne = ACCESS_ONCE(tick_nohz_enabled);
> +	tne = tick_nohz_check();
>  	if (tne != rdtp->tick_nohz_enabled_snap) {
>  		if (rcu_cpu_has_callbacks(cpu, NULL))
>  			invoke_rcu_core(); /* force nohz to see update. */
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 3612fc7..d381a22 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -361,7 +361,13 @@ void __init tick_nohz_init(void)
>  /*
>   * NO HZ enabled ?
>   */
> -int tick_nohz_enabled __read_mostly  = 1;
> +static int tick_nohz_enabled __read_mostly  = 1;
> +
> +int tick_nohz_check(void)
> +{
> +	return	tick_nohz_enabled;
> +}
> +EXPORT_SYMBOL_GPL(tick_nohz_check);
> 
>  /*
>   * Enable / Disable tickless mode
> -- 
> 1.8.4.rc0.1.g8f6a3e5.dirty
> 

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