• Christoph Lameter's avatar
    alpha: Replace __get_cpu_var · 2999a4b3
    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)
    
    CC: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
    Cc: Matt Turner <mattst88@gmail.com>
    Acked-by: default avatarRichard Henderson <rth@twiddle.net>
    Signed-off-by: default avatarChristoph Lameter <cl@linux.com>
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    2999a4b3
time.c 11.9 KB