linux-kernel - Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation

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Date:	Mon, 01 Sep 2008 11:38:29 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	mingo@...e.hu, 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, peterz@...radead.org
Subject: Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation

"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:
>  
> -#if defined(CONFIG_PREEMPT_RCU) && defined(CONFIG_NO_HZ)
> +#if defined(CONFIG_NO_HZ)
>  extern void rcu_irq_enter(void);
>  extern void rcu_irq_exit(void);
>  #else
>  # define rcu_irq_enter() do { } while (0)
>  # define rcu_irq_exit() do { } while (0)
> -#endif /* CONFIG_PREEMPT_RCU */
> +#endif /* #if defined(CONFIG_NO_HZ) */

It would be better if you hung rcu_irq_enter in the irq_enter() if 
statement that checks if the task was idle or not. This way it would
be zero overhead for interruptions of non busy CPUs, keeping 
it out of many fast paths.

Haven't read everything, sorry.

-Andi
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