[PATCH v5 19/19] sched: Add blocked_donor link to task for smarter mutex handoffs

[John Stultz]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Add link to the task this task is proxying for, and use it so we
do intellegent hand-off of the owned mutex to the task we're
running on behalf.

TODO:
* Cleanup ifdef mess

Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
Cc: Qais Yousef <qyousef@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Zimuzo Ezeozue <zezeozue@xxxxxxxxxx>
Cc: Youssef Esmat <youssefesmat@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: "Paul E . McKenney" <paulmck@xxxxxxxxxx>
Cc: kernel-team@xxxxxxxxxxx
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Signed-off-by: Connor O'Brien <connoro@xxxxxxxxxx>
[jstultz: This patch was split out from larger proxy patch]
Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
---
v5:
* Split out from larger proxy patch
---
 include/linux/sched.h  |  1 +
 kernel/fork.c          |  1 +
 kernel/locking/mutex.c | 36 +++++++++++++++++++++++++++++++++---
 kernel/sched/core.c    |  7 +++++--
 4 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3b7f26df2496..c3aa05c27816 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1140,6 +1140,7 @@ struct task_struct {
 	struct rt_mutex_waiter		*pi_blocked_on;
 #endif
 
+	struct task_struct		*blocked_donor;	/* task that is boosting us */
 	struct mutex			*blocked_on;	/* lock we're blocked on */
 	bool				blocked_on_waking; /* blocked on, but waking */
 	raw_spinlock_t			blocked_lock;
diff --git a/kernel/fork.c b/kernel/fork.c
index 5b11ead90b12..a746ac03c533 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2459,6 +2459,7 @@ __latent_entropy struct task_struct *copy_process(
 	lockdep_init_task(p);
 #endif
 
+	p->blocked_donor = NULL; /* nobody is boosting us yet */
 	p->blocked_on = NULL; /* not blocked yet */
 	p->blocked_on_waking = false; /* not blocked yet */
 
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 5125bbab4119..cf6c325515c2 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -906,7 +906,7 @@ EXPORT_SYMBOL_GPL(ww_mutex_lock_interruptible);
  */
 static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)
 {
-	struct task_struct *next = NULL;
+	struct task_struct *donor, *next = NULL;
 	DEFINE_WAKE_Q(wake_q);
 	unsigned long owner;
 	unsigned long flags;
@@ -943,7 +943,34 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 
 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
 	debug_mutex_unlock(lock);
-	if (!list_empty(&lock->wait_list)) {
+
+#ifdef CONFIG_PROXY_EXEC /* TODO: Clean up this ifdef mess */
+	raw_spin_lock(&current->blocked_lock);
+	/*
+	 * If we have a task boosting us, and that task was boosting us through
+	 * this lock, hand the lock to that task, as that is the highest
+	 * waiter, as selected by the scheduling function.
+	 */
+	donor = current->blocked_donor;
+	if (donor) {
+		struct mutex *next_lock;
+
+		raw_spin_lock_nested(&donor->blocked_lock, SINGLE_DEPTH_NESTING);
+		next_lock = get_task_blocked_on(donor);
+		if (next_lock == lock) {
+			next = donor;
+			donor->blocked_on_waking = true;
+			wake_q_add(&wake_q, donor);
+			current->blocked_donor = NULL;
+		}
+		raw_spin_unlock(&donor->blocked_lock);
+	}
+#endif
+
+	/*
+	 * Failing that, pick any on the wait list.
+	 */
+	if (!next && !list_empty(&lock->wait_list)) {
 		/* get the first entry from the wait-list: */
 		struct mutex_waiter *waiter =
 			list_first_entry(&lock->wait_list,
@@ -952,7 +979,7 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 		next = waiter->task;
 
 		debug_mutex_wake_waiter(lock, waiter);
-		raw_spin_lock(&next->blocked_lock);
+		raw_spin_lock_nested(&next->blocked_lock, SINGLE_DEPTH_NESTING);
 		WARN_ON(next->blocked_on != lock);
 		next->blocked_on_waking = true;
 		raw_spin_unlock(&next->blocked_lock);
@@ -963,6 +990,9 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 		__mutex_handoff(lock, next);
 
 	preempt_disable();
+#ifdef CONFIG_PROXY_EXEC
+	raw_spin_unlock(&current->blocked_lock);
+#endif
 	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
 
 	wake_up_q(&wake_q);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ab3b3a783ee9..59ec06988783 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6907,6 +6907,8 @@ proxy(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
 		 */
 		raw_spin_unlock(&p->blocked_lock);
 		raw_spin_unlock(&mutex->wait_lock);
+
+		owner->blocked_donor = p;
 	}
 
 	WARN_ON_ONCE(owner && !owner->on_rq);
@@ -7042,10 +7044,11 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 		switch_count = &prev->nvcsw;
 	}
 
-	proxied = (rq_selected(rq) != prev);
+	proxied = !!prev->blocked_donor;
 pick_again:
 	next = pick_next_task(rq, rq_selected(rq), &rf);
 	rq_set_selected(rq, next);
+	next->blocked_donor = NULL;
 	if (unlikely(task_is_blocked(next))) {
 		next = proxy(rq, next, &rf);
 		if (!next) {
@@ -7109,7 +7112,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 		rq = context_switch(rq, prev, next, &rf);
 	} else {
 		/* In case next was already curr but just got blocked_donor*/
-		if (unlikely(!task_current_selected(rq, next)))
+		if (unlikely(!proxied && next->blocked_donor))
 			proxy_tag_curr(rq, next);
 
 		rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
-- 
2.42.0.rc1.204.g551eb34607-goog