Subject: [PATCH 3/4] locking/rtmutex: Avoid pointless blk_flush_plug() invocations
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Thu, 27 Apr 2023 13:19:36 +0200

With DEBUG_RT_MUTEXES enabled the fast-path rt_mutex_cmpxchg_acquire()
always fails and all lock operations take the slow path, which leads to the
invocation of blk_flush_plug() even if the lock is not contended which is
unnecessary and avoids batch processing of requests.

Provide a new helper inline rt_mutex_try_acquire() which maps to
rt_mutex_cmpxchg_acquire() in the non-debug case. For the debug case it
invokes rt_mutex_slowtrylock() which can acquire a non-contended rtmutex
under full debug coverage.

Replace the rt_mutex_cmpxchg_acquire() invocations in __rt_mutex_lock() and
__ww_rt_mutex_lock() with the new helper function, which avoid the
blk_flush_plug() for the non-contended case and preserves the debug
mechanism.

[ tglx: Created a new helper and massaged changelog ]

Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Signed-off-by: Thomas Gleixner <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
---
 kernel/locking/rtmutex.c     |   25 ++++++++++++++++++++++++-
 kernel/locking/ww_rt_mutex.c |    2 +-
 2 files changed, 25 insertions(+), 2 deletions(-)

Index: linux-6.3.0-rt11/kernel/locking/rtmutex.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/locking/rtmutex.c
+++ linux-6.3.0-rt11/kernel/locking/rtmutex.c
@@ -218,6 +218,11 @@ static __always_inline bool rt_mutex_cmp
 	return try_cmpxchg_acquire(&lock->owner, &old, new);
 }
 
+static __always_inline bool rt_mutex_try_acquire(struct rt_mutex_base *lock)
+{
+	return rt_mutex_cmpxchg_acquire(lock, NULL, current);
+}
+
 static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
 						     struct task_struct *old,
 						     struct task_struct *new)
@@ -297,6 +302,24 @@ static __always_inline bool rt_mutex_cmp
 
 }
 
+static int __sched rt_mutex_slowtrylock(struct rt_mutex_base *lock);
+
+static __always_inline bool rt_mutex_try_acquire(struct rt_mutex_base *lock)
+{
+	/*
+	 * With debug enabled rt_mutex_cmpxchg trylock() will always fail,
+	 * which will unconditionally invoke sched_submit/resume_work() in
+	 * the slow path of __rt_mutex_lock() and __ww_rt_mutex_lock() even
+	 * in the non-contended case.
+	 *
+	 * Avoid that by using rt_mutex_slow_trylock() which is covered by
+	 * the debug code and can acquire a non-contended rtmutex. On
+	 * success the callsite avoids the sched_submit/resume_work()
+	 * dance.
+	 */
+	return rt_mutex_slowtrylock(lock);
+}
+
 static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
 						     struct task_struct *old,
 						     struct task_struct *new)
@@ -1704,7 +1727,7 @@ static int __sched rt_mutex_slowlock(str
 static __always_inline int __rt_mutex_lock(struct rt_mutex_base *lock,
 					   unsigned int state)
 {
-	if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
+	if (likely(rt_mutex_try_acquire(lock)))
 		return 0;
 
 	return rt_mutex_slowlock(lock, NULL, state);
Index: linux-6.3.0-rt11/kernel/locking/ww_rt_mutex.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/locking/ww_rt_mutex.c
+++ linux-6.3.0-rt11/kernel/locking/ww_rt_mutex.c
@@ -62,7 +62,7 @@ __ww_rt_mutex_lock(struct ww_mutex *lock
 	}
 	mutex_acquire_nest(&rtm->dep_map, 0, 0, nest_lock, ip);
 
-	if (likely(rt_mutex_cmpxchg_acquire(&rtm->rtmutex, NULL, current))) {
+	if (likely(rt_mutex_try_acquire(&rtm->rtmutex))) {
 		if (ww_ctx)
 			ww_mutex_set_context_fastpath(lock, ww_ctx);
 		return 0;