• Christoph Lameter's avatar
    sh: Replace __get_cpu_var uses · c473b2c6
    Christoph Lameter authored
    __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: default avatarChristoph Lameter <cl@linux.com>
    Tested-by: Geert Uytterhoeven <geert@linux-m68k.org> [compilation only]
    Cc: Paul Mundt <lethal@linux-sh.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    c473b2c6
smp.c 9.57 KB