linux-kernel - Re: [Use cpuops V1 05/11] core: Replace __get_cpu_var with __this_cpu

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Date:	Tue, 07 Dec 2010 15:29:54 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
CC:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [Use cpuops V1 05/11] core: Replace __get_cpu_var with __this_cpu_read
 if not used for an address.

Hello,

On 12/06/2010 06:16 PM, Christoph Lameter wrote:
> __get_cpu_var() can be replaced with this_cpu_read and will then use a single
> read instruction with implied address calculation to access the correct per cpu
> instance.
> 
> However, the address of a per cpu variable passed to __this_cpu_read() cannot be
> determed (since its an implied address conversion through segment prefixes).
> Therefore apply this only to uses of __get_cpu_var where the addres of the
> variable is not used.
> 
> Cc: Pekka Enberg <penberg@...helsinki.fi>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Christoph Lameter <cl@...ux.com>
...
> @@ -802,18 +802,18 @@ static void takeover_tasklets(unsigned i
>  
>  	/* Find end, append list for that CPU. */
>  	if (&per_cpu(tasklet_vec, cpu).head != per_cpu(tasklet_vec, cpu).tail) {
> -		*(__get_cpu_var(tasklet_vec).tail) = per_cpu(tasklet_vec, cpu).head;
> -		__get_cpu_var(tasklet_vec).tail = per_cpu(tasklet_vec, cpu).tail;
> +		*__this_cpu_read(tasklet_vec.tail) = per_cpu(tasklet_vec, cpu).head;
> +		this_cpu_write(tasklet_vec.tail, per_cpu(tasklet_vec, cpu).tail);
>  		per_cpu(tasklet_vec, cpu).head = NULL;
>  		per_cpu(tasklet_vec, cpu).tail = &per_cpu(tasklet_vec, cpu).head;
>  	}
>  	raise_softirq_irqoff(TASKLET_SOFTIRQ);
>  
>  	if (&per_cpu(tasklet_hi_vec, cpu).head != per_cpu(tasklet_hi_vec, cpu).tail) {
> -		*__get_cpu_var(tasklet_hi_vec).tail = per_cpu(tasklet_hi_vec, cpu).head;
> -		__get_cpu_var(tasklet_hi_vec).tail = per_cpu(tasklet_hi_vec, cpu).tail;
> +		*__this_cpuo_read(tasklet_hi_vec.tail) = per_cpu(tasklet_hi_vec, cpu).head;
> +		__this_cpu_write(tasklet_hi_vec.tail, per_cpu(tasklet_hi_vec, cpu).tail;

I don't think __this_cpuo_read() would build.

-- 
tejun
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