1. 10 8月, 2015 1 次提交
  2. 05 6月, 2014 1 次提交
    • C
      sh: Replace __get_cpu_var uses · c473b2c6
      Christoph Lameter 提交于
      __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: NChristoph Lameter <cl@linux.com>
      Tested-by: Geert Uytterhoeven <geert@linux-m68k.org> [compilation only]
      Cc: Paul Mundt <lethal@linux-sh.org>
      Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      c473b2c6
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