bpf/bcc reports error when trying to access `struct rq`

This is my bpf program to profile a kernel function pick_next_task_fiar.

#include <uapi/linux/ptrace.h>
#include <linux/sched.h>
#include <linux/nsproxy.h>
#include <linux/pid_namespace.h>


struct rq; // forward declaration

struct val_t {
   pid_t pid;
   u64 vruntime;
   int type;       // Note, 0 for previous task, 1 for next task.
};

BPF_PERF_OUTPUT(events);

int kprobe_pick_next_fair(struct pt_regs *ctx, struct rq *rq, 
struct task_struct *prev)
{


    int cpu = rq->cpu;
    struct val_t data = {};
    data.pid = prev->pid;
    data.vruntime = prev->se.vruntime;
    data.type = 0;
    events.perf_submit(ctx, &data, sizeof(data));    

    return 0;
};

It reports error as follow:

    int cpu = rq->cpu;
              ~~^
/virtual/main.c:8:8: note: forward declaration of 'struct rq'
struct rq; // forward declaration
       ^
1 error generated.
Traceback (most recent call last):
  File "picknextfair__back.py", line 73, in <module>
    b = BPF(text=bpf_text)
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 297, in __init__
    raise Exception("Failed to compile BPF text:n%s" % text)
Exception: Failed to compile BPF text:

My question why the bpf can not recognise the struct rq since I have already included the # include <linux/sched.h>. However, it does recognise the struct task_struct. These two structs are in the same head file.

Kernel version: 4.4.0-141-generic on ubuntu 16.04

Answer

struct rq is actually not part of the kernel headers, as you can see on Bootlin.

You can either:

• retrieve the offset to rq->cpu from the rq pointer and hardcode it in your BPF program, but I wouldn’t recommend it;
• or find some other way to retrieve the cpu number, maybe through prev or the current task (e.g., prev->cpu).