sched/deadline: speed up SCHED_DEADLINE 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>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1383831828-15501-14-git-send-email-juri.lelli@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/sched.h

@@ -10,6 +10,7 @@
 #include <linux/slab.h>
 
 #include "cpupri.h"
+#include "cpudeadline.h"
 #include "cpuacct.h"
 
 struct rq;
@@ -503,6 +504,7 @@ struct root_domain {
 	cpumask_var_t dlo_mask;
 	atomic_t dlo_count;
 	struct dl_bw dl_bw;
+	struct cpudl cpudl;
 
 	/*
 	 * The "RT overload" flag: it gets set if a CPU has more than