Per-CPU variables
================================================================================

Per-CPU variables are one of the kernel features. You can understand the meaning of this feature 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 an API for creating per-cpu variables - the `DEFINE_PER_CPU` macro:

```C
#define DEFINE_PER_CPU(type, name) \
        DEFINE_PER_CPU_SECTION(type, name, "")
```

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 variables, for example like this:

```C
DEFINE_PER_CPU(int, per_cpu_n)
```

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 parameters 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)    \
         __PCPU_ATTRS(sec) PER_CPU_DEF_ATTRIBUTES  \
         __typeof__(type) name
```

```C
#define __PCPU_ATTRS(sec)                                                \
         __percpu __attribute__((section(PER_CPU_BASE_SECTION sec)))     \
         PER_CPU_ATTRIBUTES
```

where `section` is:

```C
#define PER_CPU_BASE_SECTION ".data..percpu"
```

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 a `per_cpu_n` variable in the `.data..percpu` section. We can find this section in the `vmlinux`:

```
.data..percpu 00013a58  0000000000000000  0000000001a5c000  00e00000  2**12
              CONTENTS, ALLOC, LOAD, DATA
```

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 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).

```C
pr_info("NR_CPUS:%d nr_cpumask_bits:%d nr_cpu_ids:%d nr_node_ids:%d\n",
        NR_CPUS, nr_cpumask_bits, nr_cpu_ids, nr_node_ids);
```

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.

We can see this output in the dmesg:

```
$ dmesg | grep percpu
[    0.000000] setup_percpu: NR_CPUS:8 nr_cpumask_bits:8 nr_cpu_ids:8 nr_node_ids:1
```

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.
		Currently supported values are "embed" and "page".
		Archs may support subset or none of the	selections.
		See comments in mm/percpu.c for details on each
		allocator.  This parameter is primarily	for debugging
		and performance comparison.
```

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 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`:

```C
enum pcpu_fc pcpu_chosen_fc __initdata = PCPU_FC_AUTO;
```

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 above, first of all we make a check of the first chunk allocator type in the `setup_per_cpu_areas`. We check that first chunk allocator is not page:

```C
if (pcpu_chosen_fc != PCPU_FC_PAGE) {
    ...
    ...
    ...
}
```

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:

```C
rc = pcpu_embed_first_chunk(PERCPU_FIRST_CHUNK_RESERVE,
					    dyn_size, atom_size,
					    pcpu_cpu_distance,
					    pcpu_fc_alloc, pcpu_fc_free);
```

As shown above, the `pcpu_embed_first_chunk` function embeds the first percpu chunk into bootmem then we pass a couple of parameters to the `pcup_embed_first_chunk`. They are as follows:

* `PERCPU_FIRST_CHUNK_RESERVE` - the size of the reserved space for the static `percpu` variables;
* `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.

We calculate all of these parameters before the call of the `pcpu_embed_first_chunk`:

```C
const size_t dyn_size = PERCPU_MODULE_RESERVE + PERCPU_DYNAMIC_RESERVE - PERCPU_FIRST_CHUNK_RESERVE;
size_t atom_size;
#ifdef CONFIG_X86_64
		atom_size = PMD_SIZE;
#else
		atom_size = PAGE_SIZE;
#endif
```

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 an API for per-cpu variables manipulating:

* get_cpu_var(var)
* put_cpu_var(var)


Let's look at the `get_cpu_var` implementation:

```C
#define get_cpu_var(var)     \
(*({                         \
         preempt_disable();  \
         this_cpu_ptr(&var); \
}))
```

The Linux kernel is preemptible and accessing a per-cpu variable requires us to know which processor the kernel is 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 then a call of the `this_cpu_ptr` macro, which looks like:

```C
#define this_cpu_ptr(ptr) raw_cpu_ptr(ptr)
```

and

```C
#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 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);
...
//Do something with the 'var'
...
put_cpu_var(var);
```

Let's look at the `per_cpu_ptr` macro:

```C
#define per_cpu_ptr(ptr, cpu)                             \
({                                                        \
        __verify_pcpu_ptr(ptr);                           \
         SHIFT_PERCPU_PTR((ptr), per_cpu_offset((cpu)));  \
})
```

As I wrote above, this macro returns a per-cpu variable for the given cpu. First of all it calls `__verify_pcpu_ptr`:

```C
#define __verify_pcpu_ptr(ptr)
do {
	const void __percpu *__vpp_verify = (typeof((ptr) + 0))NULL;
	(void)__vpp_verify;
} while (0)
```

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. As first parameter we pass our ptr and for second parameter we pass the cpu number to the `per_cpu_offset` macro:

```C
#define per_cpu_offset(x) (__per_cpu_offset[x])
```

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. 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:

```C
#define SHIFT_PERCPU_PTR(__p, __offset)                                 \
         RELOC_HIDE((typeof(*(__p)) __kernel __force *)(__p), (__offset))
```

`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 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 a pointer to a per-cpu variable:

* 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 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.

That's all.