sched: make dl_bw a sub-quota of rt_bw

Change real-time bandwidth management as to make dl_bw a sub-quota
of rt_bw. This patch leaves rt_bw at its default value and sets
dl_bw at 40% of rt_bw. It also remove sched_dl_period_us control
knob using sched_rt_period_us as common period for both rt_bw and
dl_bw.

Checks are made when the user tries to change dl_bw sub-quota as to
not fall below what currently used. Since dl_bw now depends upon
rt_bw, similar checks are performed when the users modifies rt_bw
and dl_bw is changed accordingly. Setting rt_bw sysctl variable to
-1 (actually disabling rt throttling) disables dl_bw checks as well.

Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
sched: speed up -dl pushes with a push-heap.

Data from tests confirmed that the original active load balancing
logic didn't scale neither in the number of CPU nor in the number of
tasks (as sched_rt does).

Here we provide a global data structure to keep track of deadlines
of the running tasks in the system. The structure is composed by
a bitmask showing the free CPUs and a max-heap, needed when the system
is heavily loaded.

The implementation and concurrent access scheme are kept simple by
design. However, our measurements show that we can compete with sched_rt
on large multi-CPUs machines [1].

Only the push path is addressed, the extension to use this structure
also for pull decisions is straightforward. However, we are currently
evaluating different (in order to decrease/avoid contention) data
structures to solve possibly both problems. We are also going to re-run
tests considering recent changes inside cpupri [2].

[1] http://retis.sssup.it/~jlelli/papers/Ospert11Lelli.pdf
[2] http://www.spinics.net/lists/linux-rt-users/msg06778.html

Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
sched: add sched_dl documentation.

Add in Documentation/scheduler/ some hints about the design
choices, the usage and the future possible developments of the
sched_dl scheduling class and of the SCHED_DEADLINE policy.

Signed-off-by: Dario Faggioli <raistlin@linux.it>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>