[tip: locking/core] locking/rtmutex: Squash !RT tasks to DEFAULT_PRIO

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     715f7f9ece4685157bb59560f6c612340d730ab4
Gitweb:        https://git.kernel.org/tip/715f7f9ece4685157bb59560f6c612340d730ab4
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Sun, 15 Aug 2021 23:28:30 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 17 Aug 2021 17:51:02 +02:00

locking/rtmutex: Squash !RT tasks to DEFAULT_PRIO

Ensure all !RT tasks have the same prio such that they end up in FIFO
order and aren't split up according to nice level.

The reason why nice levels were taken into account so far is historical. In
the early days of the rtmutex code it was done to give the PI boosting and
deboosting a larger coverage.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210815211303.938676930@xxxxxxxxxxxxx
---
 kernel/locking/rtmutex.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 951bef0..ac8fb2f 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -244,11 +244,28 @@ static __always_inline bool unlock_rt_mutex_safe(struct rt_mutex_base *lock,
 }
 #endif
 
+static __always_inline int __waiter_prio(struct task_struct *task)
+{
+	int prio = task->prio;
+
+	if (!rt_prio(prio))
+		return DEFAULT_PRIO;
+
+	return prio;
+}
+
+static __always_inline void
+waiter_update_prio(struct rt_mutex_waiter *waiter, struct task_struct *task)
+{
+	waiter->prio = __waiter_prio(task);
+	waiter->deadline = task->dl.deadline;
+}
+
 /*
  * Only use with rt_mutex_waiter_{less,equal}()
  */
 #define task_to_waiter(p)	\
-	&(struct rt_mutex_waiter){ .prio = (p)->prio, .deadline = (p)->dl.deadline }
+	&(struct rt_mutex_waiter){ .prio = __waiter_prio(p), .deadline = (p)->dl.deadline }
 
 static __always_inline int rt_mutex_waiter_less(struct rt_mutex_waiter *left,
 						struct rt_mutex_waiter *right)
@@ -698,8 +715,7 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
 	 * serializes all pi_waiters access and rb_erase() does not care about
 	 * the values of the node being removed.
 	 */
-	waiter->prio = task->prio;
-	waiter->deadline = task->dl.deadline;
+	waiter_update_prio(waiter, task);
 
 	rt_mutex_enqueue(lock, waiter);
 
@@ -969,8 +985,7 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock,
 	raw_spin_lock(&task->pi_lock);
 	waiter->task = task;
 	waiter->lock = lock;
-	waiter->prio = task->prio;
-	waiter->deadline = task->dl.deadline;
+	waiter_update_prio(waiter, task);
 
 	/* Get the top priority waiter on the lock */
 	if (rt_mutex_has_waiters(lock))