I don't know how the policy SCHED_FIFO&SCHED_RR both work in kernel.
 

How the policy switch between SCHED_FIFO and SCHED_RR?

Look at sched.c.

The following 4 members belong to the task_struct,but what are the use of these four members?Process already has Pid,but ,what is the purpose that credentials used in this structure?Does anybody can give me a hand to show the use of these four members. And what is the distinction between "real_cred" and "cred"?

const struct cred *real_cred; /* objective and real subjective task
* credentials (COW) */
const struct cred *cred; /* effective (overridable) subjective task
* credentials (COW) */
struct mutex cred_guard_mutex; /* guard against foreign influences on
* credential calculations
* (notably. ptrace) */
struct cred *replacement_session_keyring; /*