linux-kernel - Re: [PATCH tip/core/rcu 1/3] rcu: Optimize rcu_is_nocb_cpu() for RCU_NOCB_CPU

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Date:	Mon, 17 Feb 2014 13:50:53 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, oleg@...hat.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 1/3] rcu: Optimize rcu_is_nocb_cpu() for
 RCU_NOCB_CPU_ALL

On Mon, Feb 17, 2014 at 01:40:25PM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> If CONFIG_RCU_NOCB_CPU_ALL=y, then rcu_is_nocb_cpu() will always
> return true, however, the current version nevertheless checks
> rcu_nocb_mask.  This commit therefore creates a static inline
> implementation of rcu_is_nocb_cpu() that unconditionally returns
> true when CONFIG_RCU_NOCB_CPU_ALL=y.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

One suggestion below.  With or without that change:
Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  include/linux/rcupdate.h | 4 ++++
>  kernel/rcu/tree_plugin.h | 2 ++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 72bf3a01a4ee..92365b1fede4 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -1016,7 +1016,11 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
>  	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
>  
>  #ifdef CONFIG_RCU_NOCB_CPU
> +#ifdef CONFIG_RCU_NOCB_CPU_ALL
> +static inline bool rcu_is_nocb_cpu(int cpu) { return true; }
> +#else /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
>  bool rcu_is_nocb_cpu(int cpu);
> +#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU_ALL */
>  #else
>  static inline bool rcu_is_nocb_cpu(int cpu) { return false; }
>  #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */

This seem clearer if written without the nesting:

#if defined(CONFIG_RCU_NOCB_CPU_ALL)
static inline bool rcu_is_nocb_cpu(int cpu) { return true; }
#elif defined(CONFIG_RCU_NOCB_CPU)
bool rcu_is_nocb_cpu(int cpu);
#else
static inline bool rcu_is_nocb_cpu(int cpu) { return false; }
#endif

That clearly distinguishes the three cases: all, some, none.

> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 6e2ef4b2b920..39a50b918bff 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2101,6 +2101,7 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
>  	init_waitqueue_head(&rnp->nocb_gp_wq[1]);
>  }
>  
> +#ifndef CONFIG_RCU_NOCB_CPU_ALL
>  /* Is the specified CPU a no-CPUs CPU? */
>  bool rcu_is_nocb_cpu(int cpu)
>  {
> @@ -2108,6 +2109,7 @@ bool rcu_is_nocb_cpu(int cpu)
>  		return cpumask_test_cpu(cpu, rcu_nocb_mask);
>  	return false;
>  }
> +#endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
>  
>  /*
>   * Enqueue the specified string of rcu_head structures onto the specified
> -- 
> 1.8.1.5
> 
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