Where could I find the code of "sched_getcpu()"

Question

Recently I'm using the function sched_getcpu() from the header file sched.h on Linux.

However, I'm wondering where could I find the source code of this function?

Thanks.

Answer 1

Under Linux, the sched_getcpu() function is a glibc wrapper to sys_getcpu() system call, which is architecture specific.

For the x86_64 architecture, it is defined under arch/x86/include/asm/vgtod.h as __getcpu() (tree 4.x):

#ifdef CONFIG_X86_64

#define VGETCPU_CPU_MASK 0xfff

static inline unsigned int __getcpu(void)
{
        unsigned int p;

        /*
         * Load per CPU data from GDT.  LSL is faster than RDTSCP and
         * works on all CPUs.  This is volatile so that it orders
         * correctly wrt barrier() and to keep gcc from cleverly
         * hoisting it out of the calling function.
         */
        asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));

        return p;
}

#endif /* CONFIG_X86_64 */

Being this function called by __vdso_getcpu() declared in arch/entry/vdso/vgetcpu.c:

notrace long
__vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused)
{
        unsigned int p;

        p = __getcpu();

        if (cpu)
                *cpu = p & VGETCPU_CPU_MASK;
        if (node)
                *node = p >> 12;
        return 0;
}

(See vDSO for details regarding what vdso prefix is).

EDIT 1: (in reply to arm code location)

ARM code location

It can be found in the arch/arm/include/asm/thread_info.h file:

static inline struct thread_info *current_thread_info(void)
{
        return (struct thread_info *)
                (current_stack_pointer & ~(THREAD_SIZE - 1));
}

This function is used by raw_smp_processor_id() that is defined in the file arch/arm/include/asm/smp.h as:

#define raw_smp_processor_id() (current_thread_info()->cpu)

And it's called by getcpu system call declared in the file kernel/sys.c:

SYSCALL_DEFINE3(getcpu, unsigned __user *, cpup, unsigned __user *, nodep, struct getcpu_cache __user *, unused)
{
        int err = 0;
        int cpu = raw_smp_processor_id();

        if (cpup)
                err |= put_user(cpu, cpup);
        if (nodep)
                err |= put_user(cpu_to_node(cpu), nodep);
        return err ? -EFAULT : 0;
}