linux-kernel - Re: [PATCH] rcu: Remove reduplicate check of cpu

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Message-ID: <20150225190028.GP15405@linux.vnet.ibm.com>
Date:	Wed, 25 Feb 2015 11:00:28 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Yao Dongdong <yaodongdong@...wei.com>
Cc:	josh@...htriplett.org, rostedt@...dmis.org,
	mathieu.desnoyers@...icios.com, laijs@...fujitsu.com,
	linux-kernel@...r.kernel.org, lizefan@...wei.com
Subject: Re: [PATCH] rcu: Remove reduplicate check of cpu_online

On Wed, Feb 25, 2015 at 05:09:46PM +0800, Yao Dongdong wrote:
> The calling function invoke_rcu_core will check it.
> 
> Signed-off-by: Yao Dongdong <yaodongdong@...wei.com>

Hmmm...  That is a fastpath, but invoke_rcu_core() is visible to the compiler,
which should inline anyway.  I took this patch for 4.1, but updated
the title and commit log as follows:

    rcu: Remove redundant check of cpu_online()
    
    Because invoke_cpu_core() checks whether the current CPU is online,
    there is no need for __call_rcu_core() to redundantly check it.
    There should not be any performance degradation because the called
    function is visible to the compiler.  This commit therefore removes
    the redundant check.

							Thanx, Paul

> ---
>  kernel/rcu/tree.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 48d640c..e5f9b7e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2741,7 +2741,7 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
>  	 * If called from an extended quiescent state, invoke the RCU
>  	 * core in order to force a re-evaluation of RCU's idleness.
>  	 */
> -	if (!rcu_is_watching() && cpu_online(smp_processor_id()))
> +	if (!rcu_is_watching())
>  		invoke_rcu_core();
> 
>  	/* If interrupts were disabled or CPU offline, don't invoke RCU core. */
> -- 
> 1.7.9.5
> 

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