sched/deadline: Add SCHED_DEADLINE inheritance logic

Some method to deal with rt-mutexes and make sched_dl interact with
the current PI-coded is needed, raising all but trivial issues, that
needs (according to us) to be solved with some restructuring of
the pi-code (i.e., going toward a proxy execution-ish implementation).

This is under development, in the meanwhile, as a temporary solution,
what this commits does is:

 - ensure a pi-lock owner with waiters is never throttled down. Instead,
   when it runs out of runtime, it immediately gets replenished and it's
   deadline is postponed;

 - the scheduling parameters (relative deadline and default runtime)
   used for that replenishments --during the whole period it holds the
   pi-lock-- are the ones of the waiting task with earliest deadline.

Acting this way, we provide some kind of boosting to the lock-owner,
still by using the existing (actually, slightly modified by the previous
commit) pi-architecture.

We would stress the fact that this is only a surely needed, all but
clean solution to the problem. In the end it's only a way to re-start
discussion within the community. So, as always, comments, ideas, rants,
etc.. are welcome! :-)

Signed-off-by: Dario Faggioli <raistlin@linux.it>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
[ Added !RT_MUTEXES build fix. ]
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1383831828-15501-11-git-send-email-juri.lelli@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 23 insertions, 9 deletions

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 3bf0aa68dd3f..2e960a2bab81 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -96,13 +96,16 @@ static inline int
 rt_mutex_waiter_less(struct rt_mutex_waiter *left,
                      struct rt_mutex_waiter *right)
 {
-        if (left->task->prio < right->task->prio)
+        if (left->prio < right->prio)
                 return 1;
 
         /*
-         * If both tasks are dl_task(), we check their deadlines.
+         * If both waiters have dl_prio(), we check the deadlines of the
+         * associated tasks.
+         * If left waiter has a dl_prio(), and we didn't return 1 above,
+         * then right waiter has a dl_prio() too.
          */
-        if (dl_prio(left->task->prio) && dl_prio(right->task->prio))
+        if (dl_prio(left->prio))
                 return (left->task->dl.deadline < right->task->dl.deadline);
 
         return 0;
@@ -197,10 +200,18 @@ int rt_mutex_getprio(struct task_struct *task)
         if (likely(!task_has_pi_waiters(task)))
                 return task->normal_prio;
 
-        return min(task_top_pi_waiter(task)->task->prio,
+        return min(task_top_pi_waiter(task)->prio,
                    task->normal_prio);
 }
 
+struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
+{
+        if (likely(!task_has_pi_waiters(task)))
+                return NULL;
+
+        return task_top_pi_waiter(task)->task;
+}
+
 /*
  * Adjust the priority of a task, after its pi_waiters got modified.
  *
@@ -210,7 +221,7 @@ static void __rt_mutex_adjust_prio(struct task_struct *task)
 {
         int prio = rt_mutex_getprio(task);
 
-        if (task->prio != prio)
+        if (task->prio != prio || dl_prio(prio))
                 rt_mutex_setprio(task, prio);
 }
 
@@ -328,7 +339,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
          * When deadlock detection is off then we check, if further
          * priority adjustment is necessary.
          */
-        if (!detect_deadlock && waiter->task->prio == task->prio)
+        if (!detect_deadlock && waiter->prio == task->prio)
                 goto out_unlock_pi;
 
         lock = waiter->lock;
@@ -350,7 +361,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
 
         /* Requeue the waiter */
         rt_mutex_dequeue(lock, waiter);
-        waiter->task->prio = task->prio;
+        waiter->prio = task->prio;
         rt_mutex_enqueue(lock, waiter);
 
         /* Release the task */
@@ -448,7 +459,7 @@ static int try_to_take_rt_mutex(struct rt_mutex *lock, struct task_struct *task,
          * 3) it is top waiter
          */
         if (rt_mutex_has_waiters(lock)) {
-                if (task->prio >= rt_mutex_top_waiter(lock)->task->prio) {
+                if (task->prio >= rt_mutex_top_waiter(lock)->prio) {
                         if (!waiter || waiter != rt_mutex_top_waiter(lock))
                                 return 0;
                 }
@@ -508,6 +519,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
         __rt_mutex_adjust_prio(task);
         waiter->task = task;
         waiter->lock = lock;
+        waiter->prio = task->prio;
 
         /* Get the top priority waiter on the lock */
         if (rt_mutex_has_waiters(lock))
@@ -653,7 +665,8 @@ void rt_mutex_adjust_pi(struct task_struct *task)
         raw_spin_lock_irqsave(&task->pi_lock, flags);
 
         waiter = task->pi_blocked_on;
-        if (!waiter || waiter->task->prio == task->prio) {
+        if (!waiter || (waiter->prio == task->prio &&
+                        !dl_prio(task->prio))) {
                 raw_spin_unlock_irqrestore(&task->pi_lock, flags);
                 return;
         }
diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index b65442fe5ade..7431a9c86f35 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -54,6 +54,7 @@ struct rt_mutex_waiter {
         struct pid              *deadlock_task_pid;
         struct rt_mutex         *deadlock_lock;
 #endif
+        int prio;
 };
 
 /*