[PATCH] locking/rtmutex: Do the trylock-slowpath with DEBUG_RT_MUTEXES enabled.

[Sebastian Andrzej Siewior]

With DEBUG_RT_MUTEXES enabled the fast-path locking
(rt_mutex_cmpxchg_acquire()) always fails. This leads to the invocation
of blk_flush_plug() even if the lock is not acquired which is
unnecessary and avoids batch processing of requests.

rt_mutex_slowtrylock() performs the trylock-slowpath and acquires the
lock if possible.
__rt_mutex_trylock() performs the fastpath try-lock and the slowpath
trylock. The latter is not desired in the non-debug case because it
fails very often even after rt_mutex_owner() reported that there is no
owner.
Here some numbers from a boot up + a few FS operations, hackbench:
- total __rt_mutex_lock() -> __rt_mutex_trylock() invocations with no
  owner: 32160
- success: 189
- failed: 31971
  - RT_MUTEX_HAS_WAITERS was set the whole time: 27469
  - owner appeared after the wait_lock has been obtained: 4502

The slowlock trylock failed in most cases without an owner because a
waiter was pending and did not acquire the lock yet. The few cases in
which it succeeded were because the pending bit was cleared after the
wait_lock was acquired.
Based on these numbers, rt_mutex_slowtrylock() in the non-DEBUG case
adds just overhead without contributing anything to the locking process.

In a dist-upgrade test with DEBUG_RT_MUTEXES enabled, the here proposed
rt_mutex_slowtrylock() optimisation acquired all locks with
current->plug set and avoided a blk_flush_plug() invocation.

Use rt_mutex_slowtrylock() in the DEBUG_RT_MUTEXES case to acquire the
lock instead the disabled rt_mutex_cmpxchg_acquire().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
On 2023-03-22 17:27:21 [+0100], To Thomas Gleixner wrote:
> > Aside of that for CONFIG_DEBUG_RT_MUTEXES=y builds it flushes on every
> > lock operation whether the lock is contended or not.
> 
> For mutex & ww_mutex operations. rwsem is not affected by
> CONFIG_DEBUG_RT_MUTEXES. As for mutex it could be mitigated by invoking
> try_to_take_rt_mutex() before blk_flush_plug().

This fixes the problem. I only observed blk_flush_plug() invocations
from down_read()/rwbase_read_lock() and down() which are not affected by
CONFIG_DEBUG_RT_MUTEXES.
I haven't observed anything in the ww-mutex path so we can ignore it or
do something similar to this.

 kernel/locking/rtmutex.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index c1bc2cb1522cb..08c599a5089a2 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1698,9 +1698,18 @@ static int __sched rt_mutex_slowlock(struct rt_mutex_base *lock,
 static __always_inline int __rt_mutex_lock(struct rt_mutex_base *lock,
 					   unsigned int state)
 {
+	/*
+	 * With DEBUG enabled cmpxchg trylock will always fail. Instead of
+	 * invoking blk_flush_plug() try the trylock-slowpath first which will
+	 * succeed if the lock is not contended.
+	 */
+#ifdef CONFIG_DEBUG_RT_MUTEXES
+	if (likely(rt_mutex_slowtrylock(lock)))
+		return 0;
+#else
 	if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
 		return 0;
-
+#endif
 	/*
 	 * If we are going to sleep and we have plugged IO queued, make sure to
 	 * submit it to avoid deadlocks.
-- 
2.40.0