[RFC PATCH 04/16] sched/core: Activate blocked donor when no owner is found

[K Prateek Nayak]

mutex_unlock_slowpath() follows:

    if (owner & MUTEX_FLAG_HANDOFF)
        break /* ... and do __mutex_handoff() */

    if (atomic_long_try_cmpxchg_release(&lock->owner, &owner, __owner_flags(owner))) {
        if (owner & MUTEX_FLAG_WAITERS)
            break; /* ... and wake up the forst waiter. */

MUTEX_FLAG_HANDOFF is only set by first-waiter after it has been woken
up and in absence of MUTEX_FLAG_HANDOFF, the owner clears itself from
the lock_word and wakes up the first waiter to try a
__mutex_trylock_or_handoff().

MUTEX_FLAG_HANDOFF exists to prevent new optimistic spinners from
trying to hijack the lock from waiter all the time and potentially
starving them but it is not necessary for MUTEX_FLAG_HANDOFF to be
always set in presence of a waiter.

If a blocked donor is deactivated when no owner is observed, it may not
be woken up until it becomes the first waiter and is naturally woken up
which breaks proxy in the interim.

Wake up the blocked donor and allow it to grab the lock when no owner is
observed. If the task manages to grab the lock, the block chain will
follow at the next proxy migration. If the task fails to grab the lock,
same situation is restored and everyone migrated to the CPU of new
owner.

Fixes: f13beb010e4a ("sched: Have try_to_wake_up() handle return-migration for PROXY_WAKING case")
Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
XXX: Is there a better way to handle this? If we can confirm a owner in
find_proxy_task(), we don't need to do a spurious wakeup of every task
observing !owner.

proxy_resched_idle() until owner appears in an option but it will spin
until next owner appears.
---
 kernel/sched/core.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4cd69b08b415..9d9db7ccf01f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6809,6 +6809,19 @@ static inline void proxy_reacquire_rq_lock(struct rq *rq, struct rq_flags *rf)
 	update_rq_clock(rq);
 }
 
+static void
+proxy_activate(struct rq *rq, struct rq_flags *rf, struct task_struct *p)
+	__must_hold(__rq_lockp(rq))
+{
+	lockdep_assert_rq_held(rq);
+	proxy_resched_idle(rq);
+	proxy_release_rq_lock(rq, rf);
+
+	wake_up_process(p);
+
+	proxy_reacquire_rq_lock(rq, rf);
+}
+
 /*
  * If the blocked-on relationship crosses CPUs, migrate @p to the
  * owner's CPU.
@@ -6934,14 +6947,15 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 			/*
 			 * If there is no owner, either clear blocked_on
 			 * and return p (if it is current and safe to
-			 * just run on this rq), or return-migrate the task.
+			 * just run on this rq), or wake the task to try
+			 * and grab the lock it is blocked on.
 			 */
 			__clear_task_blocked_on(p, NULL);
-			if (task_current(rq, p)) {
+			if (task_current(rq, p) || p->wake_cpu == task_cpu(p)) {
 				p->is_blocked = 0;
 				return p;
 			}
-			goto deactivate;
+			goto activate;
 		}
 
 		if (!READ_ONCE(owner->on_rq) || owner->se.sched_delayed) {
@@ -7029,6 +7043,9 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 	}
 	return owner;
 
+activate:
+	proxy_activate(rq, rf, p);
+	return NULL;
 deactivate:
 	proxy_deactivate(rq, p);
 	return NULL;
-- 
2.34.1