Mailing List Archive

__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.

Others usage 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 a 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 and explicit address calculation using this_cpu_ptr()
or into a use of this_cpu operations that use the offset. Thereby address calcualtions are avoided
and less registers are used when code is generated.

At the end of the patchset all uses of __get_cpu_var have been removed so the macro is removed too.

The patchset 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, u);
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(this_cpu_ptr(&y), x, 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: Christoph Lameter <cl@linux.com>

Index: linux/include/linux/kprobes.h
===================================================================
--- linux.orig/include/linux/kprobes.h 2013-08-26 14:25:53.000000000 -0500
+++ linux/include/linux/kprobes.h 2013-08-26 14:26:29.460993659 -0500
@@ -329,7 +329,7 @@ static inline void reset_current_kprobe(

static inline struct kprobe_ctlblk *get_kprobe_ctlblk(void)
{
- return (&__get_cpu_var(kprobe_ctlblk));
+ return this_cpu_ptr(&kprobe_ctlblk);
}

int register_kprobe(struct kprobe *p);
Index: linux/kernel/trace/ftrace.c
===================================================================
--- linux.orig/kernel/trace/ftrace.c 2013-08-26 14:25:53.000000000 -0500
+++ linux/kernel/trace/ftrace.c 2013-08-26 14:26:29.460993659 -0500
@@ -870,7 +870,7 @@ function_profile_call(unsigned long ip,

local_irq_save(flags);

- stat = &__get_cpu_var(ftrace_profile_stats);
+ stat = this_cpu_ptr(&ftrace_profile_stats);
if (!stat->hash || !ftrace_profile_enabled)
goto out;

@@ -901,7 +901,7 @@ static void profile_graph_return(struct
unsigned long flags;

local_irq_save(flags);
- stat = &__get_cpu_var(ftrace_profile_stats);
+ stat = this_cpu_ptr(&ftrace_profile_stats);
if (!stat->hash || !ftrace_profile_enabled)
goto out;

Index: linux/kernel/trace/trace.c
===================================================================
--- linux.orig/kernel/trace/trace.c 2013-08-26 14:25:53.000000000 -0500
+++ linux/kernel/trace/trace.c 2013-08-26 14:26:29.464993617 -0500
@@ -1676,7 +1676,7 @@ static void __ftrace_trace_stack(struct
*/
barrier();
if (use_stack == 1) {
- trace.entries = &__get_cpu_var(ftrace_stack).calls[0];
+ trace.entries = this_cpu_ptr(ftrace_stack.calls);
trace.max_entries = FTRACE_STACK_MAX_ENTRIES;

if (regs)

--
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