__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)
Signed-off-by: Christoph Lameter <c...@linux.com>
Index: linux/include/linux/percpu.h
===================================================================
--- linux.orig/include/linux/percpu.h 2013-08-26 14:29:49.000000000 -0500
+++ linux/include/linux/percpu.h 2013-08-26 14:30:18.302570608 -0500
@@ -28,7 +28,7 @@
*/
#define get_cpu_var(var) (*({ \
preempt_disable(); \
- &__get_cpu_var(var); }))
+ this_cpu_ptr(&var); }))
/*
* The weird & is necessary because sparse considers (void)(var) to be
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