linux-kernel - Re: [PATCH tip/core/rcu 2/2] rcu: Allow TREE_PREEMPT

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Date:	Sun, 27 Jan 2013 21:58:06 +1100
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 2/2] rcu: Allow TREE_PREEMPT_RCU on UP
 systems

On Sat, Jan 26, 2013 at 04:23:47PM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> The TINY_PREEMPT_RCU is complex, does not provide that much memory
> savings, and therefore TREE_PREEMPT_RCU should be used instead.  The
> systems where the difference between TINY_PREEMPT_RCU and TREE_PREEMPT_RCU
> are quite small compared to the memory footprint of CONFIG_PREEMPT.
> 
> This commit therefore takes a first step towards eliminating
> TINY_PREEMPT_RCU by allowing TREE_PREEMPT_RCU to be configured on !SMP
> systems.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@...htriplett.org>

> ---
>  init/Kconfig |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index a5e90e1..fb19b46 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -453,7 +453,7 @@ config TREE_RCU
>  
>  config TREE_PREEMPT_RCU
>  	bool "Preemptible tree-based hierarchical RCU"
> -	depends on PREEMPT && SMP
> +	depends on PREEMPT
>  	help
>  	  This option selects the RCU implementation that is
>  	  designed for very large SMP systems with hundreds or
> @@ -461,6 +461,8 @@ config TREE_PREEMPT_RCU
>  	  is also required.  It also scales down nicely to
>  	  smaller systems.
>  
> +	  Select this option if you are unsure.
> +
>  config TINY_RCU
>  	bool "UP-only small-memory-footprint RCU"
>  	depends on !PREEMPT && !SMP
> -- 
> 1.7.8
> 
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