smp_processor_id() internal

kuru · 06-28-2009, 04:48 AM

If I follow the smp_processor_id(), I reach at per_cpu__cpu_number. However, I can not find when the per_cpu__cpu_number is set

Please let me know.
Thank you for your reading.

Uncle_Theodore · 06-28-2009, 11:12 PM

I think it's in the /boot/System.map file. But your question is not very clear to me...

osor · 06-29-2009, 06:09 AM

I think the OP means where in the running of the kernel is this value set (this is not at all obvious since it is given a value through preprocessor stringification). The answer is, at least for x86(_64) systems, in arch/x86/kernel/setup_percpu.c, line 491:

Code:

                per_cpu(cpu_number, cpu) = cpu;

What looks like a function is actually a macro which assigns to the variable with identifier per_cpu__cpu_number, through some magic on lines 28-29 of the same file as well as in include/linux/percpu-defs.h and elsewhere.

kuru · 06-29-2009, 10:24 PM

First of all, thank you for your answers and I'm sorry for not to mention about my working environment. My working environment is like below:
- Linux kernel 2.6.29
- x86, 32bit machine with 2 processors

I have seen that code and feel strange because of below code.

Code:

// arch/x86/kernel/smpboot.c
void __init native_smp_prepare_boot_cpu(void)
{
	int me = smp_boot_processor_id();
	init_gdt(me);
	...
}

// arch/x86/kernel/smpcommon.c
__cpuinit void init_gdt(int cpu)
{
	...
	per_cpu(cpu_number, cpu) = cpu;
}

As you see, 'me' is per_cpu__cpu_number. In init_gdt() it sets the per_cpu__cpu_number to the 'me' again.

osor · 06-30-2009, 02:19 PM

Quote:

Originally Posted by kuru

As you see, 'me' is per_cpu__cpu_number. In init_gdt() it sets the per_cpu__cpu_number to the 'me' again.

Yes, you are correct. There may have been a few reasons for this (perhaps there was a difference in smp_processor_id, which gets the cpu_number after temporarily disabling preemption, or maybe the init_gdt function was needed elsewhere to set the cpu_number).

This, however, is irrelevant since the offending code has already been removed. Moreover, the entire function init_gdt() is now gone.