sched: add fair-user scheduler

Enable user-id based fair group scheduling. This is useful for anyone
who wants to test the group scheduler w/o having to enable
CONFIG_CGROUPS.

A separate scheduling group (i.e struct task_grp) is automatically created for 
every new user added to the system. Upon uid change for a task, it is made to 
move to the corresponding scheduling group.

A /proc tunable (/proc/root_user_share) is also provided to tune root
user's quota of cpu bandwidth.

Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

init/Kconfig

@@ -289,6 +289,19 @@ config FAIR_GROUP_SCHED
 	  This feature lets cpu scheduler recognize task groups and control cpu
 	  bandwidth allocation to such task groups.
 
+choice
+	depends on FAIR_GROUP_SCHED
+	prompt "Basis for grouping tasks"
+	default FAIR_USER_SCHED
+
+ 	config FAIR_USER_SCHED
+ 		bool "user id"
+ 		help
+ 		  This option will choose userid as the basis for grouping
+		  tasks, thus providing equal cpu bandwidth to each user.
+
+endchoice
+
 config SYSFS_DEPRECATED
 	bool "Create deprecated sysfs files"
 	default y