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.