Re: [guv v2 08/31] tracing: Replace __get_cpu_var uses

[Steven Rostedt]

On Mon, 26 Aug 2013 20:44:37 +0000
Christoph Lameter <cl@xxxxxxxxx> 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.
> 
> 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.

You should limit the width of the change log to 76 characters.

I fixed it for now.

> 
> 
> 
> 
> 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@xxxxxxxxx>
> 
> 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);
>  }

Masami,

Can you give me an Acked-by: for this.

Thanks!

-- Steve

>  
>  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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