问题
On multiprocessor, each core can have its own variables. I thought they are different variables in different addresses, although they are in same process and have the same name.
But I am wondering, how does the kernel implement this? Does it dispense a piece of memory to deposit all the percpu pointers, and every time it redirects the pointer to certain address with shift or something?
回答1:
Normal global variables are not per CPU. Automatic variables are on the stack, and different CPUs use different stack, so naturally they get separate variables.
I guess you're referring to Linux's per-CPU variable infrastructure.
Most of the magic is here (asm-generic/percpu.h
):
extern unsigned long __per_cpu_offset[NR_CPUS];
#define per_cpu_offset(x) (__per_cpu_offset[x])
/* Separate out the type, so (int[3], foo) works. */
#define DEFINE_PER_CPU(type, name) \
__attribute__((__section__(".data.percpu"))) __typeof__(type) per_cpu__##name
/* var is in discarded region: offset to particular copy we want */
#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))
#define __get_cpu_var(var) per_cpu(var, smp_processor_id())
The macro RELOC_HIDE(ptr, offset)
simply advances ptr
by the given offset in bytes (regardless of the pointer type).
What does it do?
- When defining
DEFINE_PER_CPU(int, x)
, an integer__per_cpu_x
is created in the special.data.percpu
section. - When the kernel is loaded, this section is loaded multiple times - once per CPU (this part of the magic isn't in the code above).
- The
__per_cpu_offset
array is filled with the distances between the copies. Supposing 1000 bytes of per cpu data are used,__per_cpu_offset[n]
would contain1000*n
. - The symbol
per_cpu__x
will be relocated, during load, to CPU 0'sper_cpu__x
. __get_cpu_var(x)
, when running on CPU 3, will translate to*RELOC_HIDE(&per_cpu__x, __per_cpu_offset[3])
. This starts with CPU 0'sx
, adds the offset between CPU 0's data and CPU 3's, and eventually dereferences the resulting pointer.
来源:https://stackoverflow.com/questions/16978959/how-are-percpu-pointers-implemented-in-the-linux-kernel