linux-kernel - Re: [PATCH, RFC, tip/core/rcu] scalable classic RCU implementation

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Date:	Fri, 22 Aug 2008 06:37:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	akpm@...ux-foundation.org, manfred@...orfullife.com,
	dipankar@...ibm.com, josht@...ux.vnet.ibm.com, schamp@....com,
	niv@...ibm.com, dvhltc@...ibm.com, ego@...ibm.com,
	laijs@...fujitsu.com, rostedt@...dmis.org
Subject: Re: [PATCH, RFC, tip/core/rcu] scalable classic RCU implementation


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> +#define MAX_RCU_LEVELS 3
> +#if NR_CPUS <= CONFIG_RCU_FANOUT
> +#define NUM_RCU_LEVELS 1
> +#define NUM_RCU_LEVEL_1 1
> +#define NUM_RCU_LEVEL_2 NR_CPUS
> +#define NUM_RCU_LEVEL_3 0
> +#define NUM_RCU_LEVEL_4 0
> +#define NUM_RCU_NODES NUM_RCU_LEVEL_1
> +#elif NR_CPUS <= CONFIG_RCU_FANOUT * CONFIG_RCU_FANOUT
> +#define NUM_RCU_LEVELS 2
> +#define NUM_RCU_LEVEL_1 1
> +#define NUM_RCU_LEVEL_2 \
> +	(((NR_CPUS) + (CONFIG_RCU_FANOUT) - 1) / (CONFIG_RCU_FANOUT))
> +#define NUM_RCU_LEVEL_3 NR_CPUS
> +#define NUM_RCU_LEVEL_4 0
> +#define NUM_RCU_NODES \
> +	((NUM_RCU_LEVEL_1) + (NUM_RCU_LEVEL_2))
> +#elif NR_CPUS <= CONFIG_RCU_FANOUT * CONFIG_RCU_FANOUT * CONFIG_RCU_FANOUT
> +#define NUM_RCU_LEVELS 3
> +#define RCU_FANOUT_SQ ((CONFIG_RCU_FANOUT) * (CONFIG_RCU_FANOUT))
> +#define NUM_RCU_LEVEL_1 1
> +#define NUM_RCU_LEVEL_2 \
> +	(((NR_CPUS) + (RCU_FANOUT_SQ) - 1) / (RCU_FANOUT_SQ))
> +#define NUM_RCU_LEVEL_3 \
> +	((NR_CPUS) + (CONFIG_RCU_FANOUT) - 1) / (CONFIG_RCU_FANOUT)
> +#define NUM_RCU_LEVEL_4 NR_CPUS
> +#define NUM_RCU_NODES \
> +	((NUM_RCU_LEVEL_1) + \
> +	 (NUM_RCU_LEVEL_2) + \
> +	 (NUM_RCU_LEVEL_3))
> +#else
> +#error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
> +#endif

just a quick stylistic suggestion: if feasible then such sizing ugliness 
should be hidden in a Kconfig file. (if Kconfig is capable enough for 
this that is)

	Ingo
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