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 <r...@twiddle.net> > CC: Ivan Kokshaysky <i...@jurassic.park.msu.ru> > Cc: Matt Turner <matts...@gmail.com> > Signed-off-by: Christoph Lameter <c...@linux.com>
Acked-by: Richard Henderson <r...@twiddle.net> r~ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/