linux-kernel - Re: [PATCH 37/40] alpha: Replace __get_cpu

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Date:	Thu, 19 Dec 2013 13:18:58 -0800
From:	Richard Henderson <rth@...ddle.net>
To:	Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>
CC:	akpm@...uxfoundation.org, rostedt@...dmis.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Matt Turner <mattst88@...il.com>
Subject: Re: [PATCH 37/40] alpha: Replace __get_cpu_var

On 12/19/2013 07:50 AM, Christoph Lameter wrote:
> __get_cpu_var() is used for multiple purposes in the kernel source. One of
> them is address calculation via the form &__get_cpu_var(x).  This calculates
> the address for the instance of the percpu variable of the current processor
> based on an offset.
> 
> Other use cases are for storing and retrieving data from the current
> processors percpu area.  __get_cpu_var() can be used as an lvalue when
> writing data or on the right side of an assignment.
> 
> __get_cpu_var() is defined as :
> 
> 
> #define __get_cpu_var(var) (*this_cpu_ptr(&(var)))
> 
> 
> 
> __get_cpu_var() always only does an address determination. However, store
> and retrieve operations could use a segment prefix (or global register on
> other platforms) to avoid the address calculation.
> 
> this_cpu_write() and this_cpu_read() can directly take an offset into a
> percpu area and use optimized assembly code to read and write per cpu
> variables.
> 
> 
> This patch converts __get_cpu_var into either an explicit address
> calculation using this_cpu_ptr() or into a use of this_cpu operations that
> use the offset.  Thereby address calculations are avoided and less registers
> are used when code is generated.
> 
> At the end of the patch set all uses of __get_cpu_var have been removed so
> the macro is removed too.
> 
> The patch set includes passes over all arches as well. Once these operations
> are used throughout then specialized macros can be defined in non -x86
> arches as well in order to optimize per cpu access by f.e.  using a global
> register that may be set to the per cpu base.
> 
> 
> 
> 
> Transformations done to __get_cpu_var()
> 
> 
> 1. Determine the address of the percpu instance of the current processor.
> 
> 	DEFINE_PER_CPU(int, y);
> 	int *x = &__get_cpu_var(y);
> 
>     Converts to
> 
> 	int *x = this_cpu_ptr(&y);
> 
> 
> 2. Same as #1 but this time an array structure is involved.
> 
> 	DEFINE_PER_CPU(int, y[20]);
> 	int *x = __get_cpu_var(y);
> 
>     Converts to
> 
> 	int *x = this_cpu_ptr(y);
> 
> 
> 3. Retrieve the content of the current processors instance of a per cpu
> variable.
> 
> 	DEFINE_PER_CPU(int, y);
> 	int x = __get_cpu_var(y)
> 
>    Converts to
> 
> 	int x = __this_cpu_read(y);
> 
> 
> 4. Retrieve the content of a percpu struct
> 
> 	DEFINE_PER_CPU(struct mystruct, y);
> 	struct mystruct x = __get_cpu_var(y);
> 
>    Converts to
> 
> 	memcpy(&x, this_cpu_ptr(&y), sizeof(x));
> 
> 
> 5. Assignment to a per cpu variable
> 
> 	DEFINE_PER_CPU(int, y)
> 	__get_cpu_var(y) = x;
> 
>    Converts to
> 
> 	this_cpu_write(y, x);
> 
> 
> 6. Increment/Decrement etc of a per cpu variable
> 
> 	DEFINE_PER_CPU(int, y);
> 	__get_cpu_var(y)++
> 
>    Converts to
> 
> 	this_cpu_inc(y)
> 
> Cc: Richard Henderson <rth@...ddle.net>
> CC: Ivan Kokshaysky <ink@...assic.park.msu.ru>
> Cc: Matt Turner <mattst88@...il.com>
> Signed-off-by: Christoph Lameter <cl@...ux.com>

Acked-by: Richard Henderson <rth@...ddle.net>


r~
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