Per-CPU variables are one of the kernel features. You can understand what this feature means by reading its name. We can create a variable and each processor core will have its own copy of this variable. We take a closer look on this feature and try to understand how it is implemented and how it works in this part.
Per-CPU variables are one of the kernel features. You can understand what this feature means by reading its name. We can create a variable and each processor core will have its own copy of this variable. In this part, we take a closer look at this feature and try to understand how it is implemented and how it works.
The kernel provides API for creating per-cpu variables - `DEFINE_PER_CPU` macro:
The kernel provides an API for creating per-cpu variables - the`DEFINE_PER_CPU` macro:
```C
#define DEFINE_PER_CPU(type, name) \
@ -12,13 +12,13 @@ The kernel provides API for creating per-cpu variables - `DEFINE_PER_CPU` macro:
This macro defined in the [include/linux/percpu-defs.h](https://github.com/torvalds/linux/blob/master/include/linux/percpu-defs.h) as many other macros for work with per-cpu variables. Now we will see how this feature is implemented.
Take a look at the `DECLARE_PER_CPU` definition. We see that it takes 2 parameters: `type` and `name`, so we can use it to create per-cpu variable, for example like this:
Take a look at the `DECLARE_PER_CPU` definition. We see that it takes 2 parameters: `type` and `name`, so we can use it to create per-cpu variables, for example like this:
```C
DEFINE_PER_CPU(int, per_cpu_n)
```
We pass the type and the name of our variable. `DEFI_PER_CPU` calls `DEFINE_PER_CPU_SECTION` macro and passes the same two paramaters and empty string to it. Let's look at the definition of the `DEFINE_PER_CPU_SECTION`:
We pass the type and the name of our variable. `DEFINE_PER_CPU` calls the`DEFINE_PER_CPU_SECTION` macro and passes the same two paramaters and empty string to it. Let's look at the definition of the `DEFINE_PER_CPU_SECTION`:
```C
#define DEFINE_PER_CPU_SECTION(type, name, sec) \
@ -32,35 +32,35 @@ We pass the type and the name of our variable. `DEFI_PER_CPU` calls `DEFINE_PER_
PER_CPU_ATTRIBUTES
```
where section is:
where `section` is:
```C
#define PER_CPU_BASE_SECTION ".data..percpu"
```
After all macros are expanded we will get global per-cpu variable:
After all macros are expanded we will get a global per-cpu variable:
```C
__attribute__((section(".data..percpu"))) int per_cpu_n
```
It means that we will have `per_cpu_n` variable in the `.data..percpu` section. We can find this section in the `vmlinux`:
It means that we will have a `per_cpu_n` variable in the `.data..percpu` section. We can find this section in the `vmlinux`:
Ok, now we know that when we use `DEFINE_PER_CPU` macro, per-cpu variable in the `.data..percpu` section will be created. When the kernel initilizes it calls `setup_per_cpu_areas` function which loads `.data..percpu` section multiply times, one section per CPU.
Ok, now we know that when we use the `DEFINE_PER_CPU` macro, a per-cpu variable in the `.data..percpu` section will be created. When the kernel initializes it calls the `setup_per_cpu_areas` function which loads the `.data..percpu` section multiple times, one section per CPU.
Let's look on the per-CPU areas initialization process. It start in the [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c) from the call of the `setup_per_cpu_areas` function which defined in the [arch/x86/kernel/setup_percpu.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup_percpu.c).
Let's look at the per-CPU areas initialization process. It starts in the [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c) from the call of the `setup_per_cpu_areas` function which is defined in the [arch/x86/kernel/setup_percpu.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup_percpu.c).
The `setup_per_cpu_areas` starts from the output information about the Maximum number of CPUs set during kernel configuration with `CONFIG_NR_CPUS` configuration option, actual number of CPUs, `nr_cpumask_bits` is the same that `NR_CPUS` bit for the new `cpumask` operators and number of `NUMA` nodes.
The `setup_per_cpu_areas` starts from the output information about the maximum number of CPUs set during kernel configuration with the`CONFIG_NR_CPUS` configuration option, actual number of CPUs, `nr_cpumask_bits` is the same that `NR_CPUS` bit for the new `cpumask` operators and number of `NUMA` nodes.
In the next step we check `percpu` first chunk allocator. All percpu areas are allocated in chunks. First chunk is used for the static percpu variables. Linux kernel has `percpu_alloc` command line parameters which provides type of the first chunk allocator. We can read about it in the kernel documentation:
In the next step we check the `percpu` first chunk allocator. All percpu areas are allocated in chunks. The first chunk is used for the static percpu variables. The Linux kernel has `percpu_alloc` command line parameters which provides the type of the first chunk allocator. We can read about it in the kernel documentation:
```
percpu_alloc= Select which percpu first chunk allocator to use.
@ -80,21 +80,21 @@ percpu_alloc= Select which percpu first chunk allocator to use.
and performance comparison.
```
The [mm/percpu.c](https://github.com/torvalds/linux/blob/master/mm/percpu.c) contains handler of this command line option:
The [mm/percpu.c](https://github.com/torvalds/linux/blob/master/mm/percpu.c) contains the handler of this command line option:
```C
early_param("percpu_alloc", percpu_alloc_setup);
```
Where `percpu_alloc_setup` function sets the `pcpu_chosen_fc` variable depends on the `percpu_alloc` parameter value. By default first chunk allocator is `auto`:
Where the `percpu_alloc_setup` function sets the `pcpu_chosen_fc` variable depends on the `percpu_alloc` parameter value. By default the first chunk allocator is `auto`:
If `percpu_alooc` parameter not given to the kernel command line, the `embed` allocator will be used wich as you can understand embed the first percpu chunk into bootmem with the [memblock](http://0xax.gitbooks.io/linux-insides/content/mm/linux-mm-1.html). The last allocator is first chunk `page` allocator which maps first chunk with `PAGE_SIZE` pages.
If the `percpu_alloc` parameter is not given to the kernel command line, the `embed` allocator will be used which embeds the first percpu chunk into bootmem with the [memblock](http://0xax.gitbooks.io/linux-insides/content/mm/linux-mm-1.html). The last allocator is the first chunk `page` allocator which maps the first chunk with `PAGE_SIZE` pages.
As I wrote about first of all we make a check of the first chunk allocator type in the `setup_per_cpu_areas`. First of all we check that first chunk allocator is not page:
As I wrote about first of all, we make a check of the first chunk allocator type in the `setup_per_cpu_areas`. First of all we check that first chunk allocator is not page:
```C
if (pcpu_chosen_fc != PCPU_FC_PAGE) {
@ -104,7 +104,7 @@ if (pcpu_chosen_fc != PCPU_FC_PAGE) {
}
```
If it is not `PCPU_FC_PAGE`, we will use `embed` allocator and allocate space for the first chunk with the `pcpu_embed_first_chunk` function:
If it is not `PCPU_FC_PAGE`, we will use the `embed` allocator and allocate space for the first chunk with the `pcpu_embed_first_chunk` function:
As I wrote above, the `pcpu_embed_first_chunk` function embeds the first percpu chunk into bootmem. As you can see we pass a couple of parameters to the `pcup_embed_first_chunk`, they are
* `PERCPU_FIRST_CHUNK_RESERVE` - the size of the reserved space for the static `percpu` variables;
* `dyn_size` - minimum free size for dynamic allocation in byte;
* `dyn_size` - minimum free size for dynamic allocation in bytes;
* `atom_size` - all allocations are whole multiples of this and aligned to this parameter;
* `pcpu_cpu_distance` - callback to determine distance between cpus;
* `pcpu_fc_alloc` - function to allocate `percpu` page;
* `pcpu_fc_free` - function to release `percpu` page.
All of this parameters we calculat before the call of the `pcpu_embed_first_chunk`:
All of these parameters we calculate before the call of the `pcpu_embed_first_chunk`:
If first chunk allocator is `PCPU_FC_PAGE`, we will use the `pcpu_page_first_chunk` instead of the `pcpu_embed_first_chunk`. After that `percpu` areas up, we setup `percpu` offset and its segment for the every CPU with the `setup_percpu_segment` function (only for `x86` systems) and move some early data from the arrays to the `percpu` variables (`x86_cpu_to_apicid`, `irq_stack_ptr` and etc...). After the kernel finished the initialization process, we have loaded N `.data..percpu` sections, where N is the number of CPU, and section used by bootstrap processor will contain uninitialized variable created with `DEFINE_PER_CPU` macro.
If the first chunk allocator is `PCPU_FC_PAGE`, we will use the `pcpu_page_first_chunk` instead of the `pcpu_embed_first_chunk`. After that `percpu` areas up, we setup `percpu` offset and its segment for every CPU with the `setup_percpu_segment` function (only for `x86` systems) and move some early data from the arrays to the `percpu` variables (`x86_cpu_to_apicid`, `irq_stack_ptr` and etc...). After the kernel finishes the initialization process, we will have loaded N `.data..percpu` sections, where N is the number of CPUs, and the section used by the bootstrap processor will contain an uninitialized variable created with the`DEFINE_PER_CPU` macro.
The kernel provides API for per-cpu variables manipulating:
The kernel provides an API for per-cpu variables manipulating:
* get_cpu_var(var)
* put_cpu_var(var)
Let's look at `get_cpu_var` implementation:
Let's look at the `get_cpu_var` implementation:
```C
#define get_cpu_var(var) \
@ -152,7 +152,7 @@ Let's look at `get_cpu_var` implementation:
}))
```
Linux kernel is preemptible and accessing a per-cpu variable requires to know which processor kernel running on. So, current code must not be preempted and moved to the another CPU while accessing a per-cpu variable. That's why first of all we can see call of the `preempt_disable` function. After this we can see call of the `this_cpu_ptr` macro, which looks as:
The Linux kernel is preemptible and accessing a per-cpu variable requires us to know which processor the kernel running on. So, current code must not be preempted and moved to the another CPU while accessing a per-cpu variable. That's why first of all we can see a call of the `preempt_disable` function. After this we can see a call of the `this_cpu_ptr` macro, which looks like:
```C
#define this_cpu_ptr(ptr) raw_cpu_ptr(ptr)
@ -164,7 +164,7 @@ and
#define raw_cpu_ptr(ptr) per_cpu_ptr(ptr, 0)
```
where `per_cpu_ptr` returns a pointer to the per-cpu variable for the given cpu (second parameter). After that we got per-cpu variables and made any manipulations on it, we must call`put_cpu_var` macro which enables preemption with call of `preempt_enable` function. So the typical usage of a per-cpu variable is following:
where `per_cpu_ptr` returns a pointer to the per-cpu variable for the given cpu (second parameter). After we've created a per-cpu variable and made modifications to it, we must call the`put_cpu_var` macro which enables preemption with a call of `preempt_enable` function. So the typical usage of a per-cpu variable is as follows:
```C
get_cpu_var(var);
@ -174,7 +174,7 @@ get_cpu_var(var);
put_cpu_var(var);
```
Let's look at `per_cpu_ptr` macro:
Let's look at the `per_cpu_ptr` macro:
```C
#define per_cpu_ptr(ptr, cpu) \
@ -184,47 +184,47 @@ Let's look at `per_cpu_ptr` macro:
})
```
As I wrote above, this macro returns per-cpu variable for the given cpu. First of all it calls `__verify_pcpu_ptr`:
As I wrote above, this macro returns a per-cpu variable for the given cpu. First of all it calls `__verify_pcpu_ptr`:
which makes given `ptr` type of `const void __percpu *`,
which makes the given `ptr` type of `const void __percpu *`,
After this we can see the call of the `SHIFT_PERCPU_PTR` macro with two parameters. At first parameter we pass our ptr and sencond we pass cpu number to the `per_cpu_offset` macro which:
After this we can see the call of the `SHIFT_PERCPU_PTR` macro with two parameters. At first parameter we pass our ptr and second we pass the cpu number to the `per_cpu_offset` macro:
```C
#define per_cpu_offset(x) (__per_cpu_offset[x])
```
expands to getting `x` element from the `__per_cpu_offset` array:
which expands to getting the`x` element from the `__per_cpu_offset` array:
```C
extern unsigned long __per_cpu_offset[NR_CPUS];
```
where `NR_CPUS` is the number of CPUs. `__per_cpu_offset` array filled with the distances between cpu-variables copies. For example all per-cpu data is `X` bytes size, so if we access `__per_cpu_offset[Y]`, so`X*Y` will be accessed. Let's look at the `SHIFT_PERCPU_PTR` implementation:
where `NR_CPUS` is the number of CPUs. The `__per_cpu_offset` array is filled with the distances between cpu-variable copies. For example all per-cpu data is `X` bytes in size, so if we access `__per_cpu_offset[Y]`, `X*Y` will be accessed. Let's look at the `SHIFT_PERCPU_PTR` implementation:
`RELOC_HIDE` just returns offset `(typeof(ptr)) (__ptr + (off))` and it will be pointer of the variable.
`RELOC_HIDE` just returns offset `(typeof(ptr)) (__ptr + (off))` and it will return a pointer to the variable.
That's all! Of course it is not the full API, but the general part. It can be hard for the start, but to understand per-cpu variables feature need to understand mainly [include/linux/percpu-defs.h](https://github.com/torvalds/linux/blob/master/include/linux/percpu-defs.h) magic.
That's all! Of course it is not the full API, but a general overview. It can be hard to start with, but to understand per-cpu variables you mainly need to understand the [include/linux/percpu-defs.h](https://github.com/torvalds/linux/blob/master/include/linux/percpu-defs.h) magic.
Let's again look at the algorithm of getting pointer on per-cpu variable:
Let's again look at the algorithm of getting a pointer to a per-cpu variable:
* The kernel creates multiply `.data..percpu` sections (ones perc-pu) during initialization process;
* All variables created with the `DEFINE_PER_CPU` macro will be reloacated to the first section or for CPU0;
* The kernel creates multiple `.data..percpu` sections (one per-cpu) during initialization process;
* All variables created with the `DEFINE_PER_CPU` macro will be relocated to the first section or for CPU0;
* `__per_cpu_offset` array filled with the distance (`BOOT_PERCPU_OFFSET`) between `.data..percpu` sections;
* When `per_cpu_ptr` called for example for getting pointer on the certain per-cpu variable for the third CPU, `__per_cpu_offset` array will be accessed, where every index points to the certain CPU.
* When the `per_cpu_ptr` is called, for example for getting a pointer on a certain per-cpu variable for the third CPU, the `__per_cpu_offset` array will be accessed, where every index points to the required CPU.
Dave Willmer
9 years ago