linux/arch/arc
Christoph Lameter 6855e95ce3 arc: Replace __get_cpu_var uses
__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 calcualtions are avoided
and less registers are used when code is generated.

At the end of the patchset all uses of __get_cpu_var have been removed so the macro is removed too.

The patchset 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, u);
	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(this_cpu_ptr(&x), 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)

Acked-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
2013-11-06 10:40:37 +05:30
..
boot ARC: Add some .gitignore entries 2013-08-26 09:40:24 +05:30
configs Couple of Platform updates (Device Tree files primarily) given that the 2013-07-10 10:11:26 -07:00
include ARC: Workaround spinlock livelock in SMP SystemC simulation 2013-09-27 16:28:48 +05:30
kernel arc: Replace __get_cpu_var uses 2013-11-06 10:40:37 +05:30
lib ARC: [lib] strchr breakage in Big-endian configuration 2013-08-24 11:24:53 -07:00
mm ARC: Incorrect mm reference used in vmalloc fault handler 2013-11-02 10:27:04 -07:00
oprofile ARC: OProfile support 2013-02-15 23:16:00 +05:30
plat-arcfpga Couple of Platform updates (Device Tree files primarily) given that the 2013-07-10 10:11:26 -07:00
plat-tb10x ARC: [TB10x] Updates for irqchip driver 2013-06-28 15:07:42 +05:30
Kbuild ARC: Build system: Makefiles, Kconfig, Linker script 2013-02-11 20:00:25 +05:30
Kconfig Remove GENERIC_HARDIRQ config option 2013-09-13 15:09:52 +02:00
Kconfig.debug consolidate per-arch stack overflow debugging options 2013-07-04 11:25:39 -07:00
Makefile ARC: Adjustments for gcc 4.8 2013-06-27 14:35:32 +05:30