Re: [PATCH 37/40] alpha: Replace __get_cpu_var
From: Richard Henderson
Date: Thu Dec 19 2013 - 16:19:11 EST
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@xxxxxxxxxxx>
> CC: Ivan Kokshaysky <ink@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matt Turner <mattst88@xxxxxxxxx>
> Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>
Acked-by: Richard Henderson <rth@xxxxxxxxxxx>
r~
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/