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Date: Tue, 19 Dec 2023 16:18:18 -0800
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Subject: [PATCH v7 07/23] locking/mutex: Switch to mutex handoffs for CONFIG_SCHED_PROXY_EXEC
From: John Stultz <jstultz@google.com>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Joel Fernandes <joelaf@google.com>, 
	Qais Yousef <qyousef@google.com>, Ingo Molnar <mingo@redhat.com>, 
	Juri Lelli <juri.lelli@redhat.com>, Vincent Guittot <vincent.guittot@linaro.org>, 
	Dietmar Eggemann <dietmar.eggemann@arm.com>, Valentin Schneider <vschneid@redhat.com>, 
	Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>, 
	Zimuzo Ezeozue <zezeozue@google.com>, Youssef Esmat <youssefesmat@google.com>, 
	Mel Gorman <mgorman@suse.de>, Daniel Bristot de Oliveira <bristot@redhat.com>, Will Deacon <will@kernel.org>, 
	Waiman Long <longman@redhat.com>, Boqun Feng <boqun.feng@gmail.com>, 
	"Paul E. McKenney" <paulmck@kernel.org>, Metin Kaya <Metin.Kaya@arm.com>, 
	Xuewen Yan <xuewen.yan94@gmail.com>, K Prateek Nayak <kprateek.nayak@amd.com>, 
	Thomas Gleixner <tglx@linutronix.de>, kernel-team@android.com, 
	Valentin Schneider <valentin.schneider@arm.com>, "Connor O'Brien" <connoro@google.com>, 
	John Stultz <jstultz@google.com>
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From: Peter Zijlstra <peterz@infradead.org>

Since with SCHED_PROXY_EXEC, we will want to hand off locks to
the tasks we are running on behalf of, switch to using mutex
handoffs.

Cc: Joel Fernandes <joelaf@google.com>
Cc: Qais Yousef <qyousef@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Youssef Esmat <youssefesmat@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-team@android.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[rebased, added comments and changelog]
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
[Fixed rebase conflicts]
[squashed sched: Ensure blocked_on is always guarded by blocked_lock]
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
[fix rebase conflicts, various fixes & tweaks commented inline]
[squashed sched: Use rq->curr vs rq->proxy checks]
Signed-off-by: Connor O'Brien <connoro@google.com>
[jstultz: Split out only the very basic initial framework
 for proxy logic from a larger patch.]
Signed-off-by: John Stultz <jstultz@google.com>
---
v5:
* Split out from core proxy patch
v6:
* Rework to use sched_proxy_exec() instead of #ifdef CONFIG_PROXY_EXEC
v7:
* Avoid disabling optimistic spinning at compile time so booting
  with sched_proxy_exec=off matches prior performance
* Add comment in mutex-design.rst as suggested by Metin Kaya
---
 Documentation/locking/mutex-design.rst |  3 ++
 kernel/locking/mutex.c                 | 42 +++++++++++++++-----------
 2 files changed, 28 insertions(+), 17 deletions(-)

diff --git a/Documentation/locking/mutex-design.rst b/Documentation/locking/mutex-design.rst
index 78540cd7f54b..57a5cb03f409 100644
--- a/Documentation/locking/mutex-design.rst
+++ b/Documentation/locking/mutex-design.rst
@@ -61,6 +61,9 @@ taken, depending on the state of the lock:
      waiting to spin on mutex owner, only to go directly to slowpath upon
      obtaining the MCS lock.
 
+     NOTE: Optimistic spinning will be avoided when using proxy execution
+     (SCHED_PROXY_EXEC) as we want to hand the lock off to the task that was
+     boosting the current owner.
 
 (iii) slowpath: last resort, if the lock is still unable to be acquired,
       the task is added to the wait-queue and sleeps until woken up by the
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 6084470773f6..11dc5cb7a5a3 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -416,6 +416,9 @@ static __always_inline bool
 mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
 		      struct mutex_waiter *waiter)
 {
+	if (sched_proxy_exec())
+		return false;
+
 	if (!waiter) {
 		/*
 		 * The purpose of the mutex_can_spin_on_owner() function is
@@ -914,26 +917,31 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 
 	mutex_release(&lock->dep_map, ip);
 
-	/*
-	 * Release the lock before (potentially) taking the spinlock such that
-	 * other contenders can get on with things ASAP.
-	 *
-	 * Except when HANDOFF, in that case we must not clear the owner field,
-	 * but instead set it to the top waiter.
-	 */
-	owner = atomic_long_read(&lock->owner);
-	for (;;) {
-		MUTEX_WARN_ON(__owner_task(owner) != current);
-		MUTEX_WARN_ON(owner & MUTEX_FLAG_PICKUP);
-
-		if (owner & MUTEX_FLAG_HANDOFF)
-			break;
+	if (sched_proxy_exec()) {
+		/* Always force HANDOFF for Proxy Exec for now. Revisit. */
+		owner = MUTEX_FLAG_HANDOFF;
+	} else {
+		/*
+		 * Release the lock before (potentially) taking the spinlock
+		 * such that other contenders can get on with things ASAP.
+		 *
+		 * Except when HANDOFF, in that case we must not clear the
+		 * owner field, but instead set it to the top waiter.
+		 */
+		owner = atomic_long_read(&lock->owner);
+		for (;;) {
+			MUTEX_WARN_ON(__owner_task(owner) != current);
+			MUTEX_WARN_ON(owner & MUTEX_FLAG_PICKUP);
 
-		if (atomic_long_try_cmpxchg_release(&lock->owner, &owner, __owner_flags(owner))) {
-			if (owner & MUTEX_FLAG_WAITERS)
+			if (owner & MUTEX_FLAG_HANDOFF)
 				break;
 
-			return;
+			if (atomic_long_try_cmpxchg_release(&lock->owner, &owner,
+							    __owner_flags(owner))) {
+				if (owner & MUTEX_FLAG_WAITERS)
+					break;
+				return;
+			}
 		}
 	}
 
-- 
2.43.0.472.g3155946c3a-goog