rtmutex: Turn the plist into an rb-tree

Turn the pi-chains from plist to rb-tree, in the rt_mutex code,
and provide a proper comparison function for -deadline and
-priority tasks.

This is done mainly because:
 - classical prio field of the plist is just an int, which might
   not be enough for representing a deadline;
 - manipulating such a list would become O(nr_deadline_tasks),
   which might be to much, as the number of -deadline task increases.

Therefore, an rb-tree is used, and tasks are queued in it according
to the following logic:
 - among two -priority (i.e., SCHED_BATCH/OTHER/RR/FIFO) tasks, the
   one with the higher (lower, actually!) prio wins;
 - among a -priority and a -deadline task, the latter always wins;
 - among two -deadline tasks, the one with the earliest deadline
   wins.

Queueing and dequeueing functions are changed accordingly, for both
the list of a task's pi-waiters and the list of tasks blocked on
a pi-lock.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Dario Faggioli <raistlin@linux.it>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
Signed-off-again-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1383831828-15501-10-git-send-email-juri.lelli@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 19 insertions, 13 deletions

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index b0ed422e4e4a..f0e52383a001 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -11,6 +11,7 @@
 #include <linux/user_namespace.h>
 #include <linux/securebits.h>
 #include <linux/seqlock.h>
+#include <linux/rbtree.h>
 #include <net/net_namespace.h>
 #include <linux/sched/rt.h>
 
@@ -154,6 +155,14 @@ extern struct task_group root_task_group;
 
 #define INIT_TASK_COMM "swapper"
 
+#ifdef CONFIG_RT_MUTEXES
+# define INIT_RT_MUTEXES(tsk)                                           \
+        .pi_waiters = RB_ROOT,                                          \
+        .pi_waiters_leftmost = NULL,
+#else
+# define INIT_RT_MUTEXES(tsk)
+#endif
+
 /*
  *  INIT_TASK is used to set up the first task table, touch at
  * your own risk!. Base=0, limit=0x1fffff (=2MB)
@@ -221,6 +230,7 @@ extern struct task_group root_task_group;
         INIT_TRACE_RECURSION                                            \
         INIT_TASK_RCU_PREEMPT(tsk)                                      \
         INIT_CPUSET_SEQ(tsk)                                            \
+        INIT_RT_MUTEXES(tsk)                                            \
         INIT_VTIME(tsk)                                                 \
 }
 
diff --git a/include/linux/rtmutex.h b/include/linux/rtmutex.h
index de17134244f3..3aed8d737e1a 100644
--- a/include/linux/rtmutex.h
+++ b/include/linux/rtmutex.h
@@ -13,7 +13,7 @@
 #define __LINUX_RT_MUTEX_H
 
 #include <linux/linkage.h>
-#include <linux/plist.h>
+#include <linux/rbtree.h>
 #include <linux/spinlock_types.h>
 
 extern int max_lock_depth; /* for sysctl */
@@ -22,12 +22,14 @@ extern int max_lock_depth; /* for sysctl */
  * The rt_mutex structure
  *
  * @wait_lock:  spinlock to protect the structure
- * @wait_list:  pilist head to enqueue waiters in priority order
+ * @waiters:    rbtree root to enqueue waiters in priority order
+ * @waiters_leftmost: top waiter
  * @owner:      the mutex owner
  */
 struct rt_mutex {
         raw_spinlock_t          wait_lock;
-        struct plist_head       wait_list;
+        struct rb_root          waiters;
+        struct rb_node          *waiters_leftmost;
         struct task_struct      *owner;
 #ifdef CONFIG_DEBUG_RT_MUTEXES
         int                     save_state;
@@ -66,7 +68,7 @@ struct hrtimer_sleeper;
 
 #define __RT_MUTEX_INITIALIZER(mutexname) \
         { .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(mutexname.wait_lock) \
-        , .wait_list = PLIST_HEAD_INIT(mutexname.wait_list) \
+        , .waiters = RB_ROOT \
         , .owner = NULL \
         __DEBUG_RT_MUTEX_INITIALIZER(mutexname)}
 
@@ -98,12 +100,4 @@ extern int rt_mutex_trylock(struct rt_mutex *lock);
 
 extern void rt_mutex_unlock(struct rt_mutex *lock);
 
-#ifdef CONFIG_RT_MUTEXES
-# define INIT_RT_MUTEXES(tsk)                                           \
-        .pi_waiters     = PLIST_HEAD_INIT(tsk.pi_waiters),      \
-        INIT_RT_MUTEX_DEBUG(tsk)
-#else
-# define INIT_RT_MUTEXES(tsk)
-#endif
-
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 158f4c2dd852..9ea15019a5b6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -16,6 +16,7 @@ struct sched_param {
 #include <linux/types.h>
 #include <linux/timex.h>
 #include <linux/jiffies.h>
+#include <linux/plist.h>
 #include <linux/rbtree.h>
 #include <linux/thread_info.h>
 #include <linux/cpumask.h>
@@ -1354,7 +1355,8 @@ struct task_struct {
 
 #ifdef CONFIG_RT_MUTEXES
         /* PI waiters blocked on a rt_mutex held by this task */
-        struct plist_head pi_waiters;
+        struct rb_root pi_waiters;
+        struct rb_node *pi_waiters_leftmost;
         /* Deadlock detection and priority inheritance handling */
         struct rt_mutex_waiter *pi_blocked_on;
 #endif