how to use printk()?

Question

it is the first time working in a Linux environment. i need a lot of help from you. i want to add prink() in shced_setattr https://elixir.bootlin.com/linux/v4.18/source/kernel/sched/core.c#L4578

SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
                   unsigned int, flags)
{
    printk();
    struct sched_attr attr;
    struct task_struct *p;
    int retval;

    if (!uattr || pid < 0 || flags)
        return -EINVAL;

    retval = sched_copy_attr(uattr, &attr);
    if (retval)
        return retval;

    if ((int)attr.sched_policy < 0)
        return -EINVAL;

    rcu_read_lock();
    retval = -ESRCH;
    p = find_process_by_pid(pid);
    if (p != NULL)
        retval = sched_setattr(p, &attr);
    rcu_read_unlock();

    return retval;
}

but i don't know how to edit the procedure. If someone could explain me edit the linux function for the printk() function it would be great!

Answer 1

It all depends on what you're trying to print using printk, which is similar to printf. The printk documentation can be found at:

https://www.kernel.org/doc/html/latest/core-api/printk-formats.html

It's also part of the source:

Documentation/printk-formats.txt

The simplest call would only involve a format string without placeholders:

printk("sched_setattr called\n");

Printing that with pid would result in:

printk("sched_setattr called: pid: %d\n", pid);

The output can be read using dmesg.

printk also supports multiple log levels, but that shouldn't be relevant for getting started.