[PATCH 3/3] Mutex: Avoid useless spinning

[Kemi Wang]

Usually, we can't set too short time out while spinning on the lock, that
probably makes a thread which is trying to acquire the lock go to sleep
quickly, thus weakens the benefit of pthread adaptive spin lock.

However, there is also a problem if we set the time out large in
case of protecting a small critical section with severe lock contention.
As we can see the test result in the last patch, the performance is highly
effected by the spin count tunables, smaller spin count, better performance
improvement. This is because the thread probably spins on the lock until
timeout in severe lock contention before going to sleep.

In this patch, we avoid the useless spin by making the spinner sleep
if it fails to acquire the lock when the lock is available, as suggested
by Tim Chen.

nr_threads    base       COUNT=1000(head~1)   COUNT=1000(head)
1           51644585      51323778(-0.6%)	     51378551(-0.5%)
2           7914789       9867343(+24.7%)	     11503559(+45.3%)
7           1687620       3430504(+103.3%)	     7817383(+363.2%)
14          1026555       1843458(+79.6%)	     7360883(+617.0%)
28          962001        681965(-29.1%)	     5681945(+490.6%)
56          883770        364879(-58.7%)	     3416068(+286.5%)
112         1150589       415261(-63.9%)	     3255700(+183.0%)

Suggested-by: Tim Chen <tim.c.chen@intel.com>
Signed-off-by: Kemi Wang <kemi.wang@intel.com>
---
 nptl/pthread_mutex_lock.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index c3aca93..0faee1a 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -127,22 +127,19 @@ __pthread_mutex_lock (pthread_mutex_t *mutex)
 	  int cnt = 0;
 	  int max_cnt = MIN (__mutex_aconf.spin_count,
 			mutex->__data.__spins * 2 + 100);
+
+     	/* MO read while spinning */
 	  do
-	    {
-		if (cnt >= max_cnt)
-		  {
-		    LLL_MUTEX_LOCK (mutex);
-		    break;
-		  }
-		/* MO read while spinning */
-		do
-		  {
-		    atomic_spin_nop ();
-		  }
-		while (atomic_load_relaxed (&mutex->__data.__lock) != 0 &&
+    	{
+		  atomic_spin_nop ();
+		}
+	  while (atomic_load_relaxed (&mutex->__data.__lock) != 0 &&
 			++cnt < max_cnt);
-	    }
-	  while (LLL_MUTEX_TRYLOCK (mutex) != 0);
+	    /* Try to acquire the lock if lock is available or the spin count
+	     * is run out, go to sleep if fails
+	     */
+	  if (LLL_MUTEX_TRYLOCK (mutex) != 0)
+		  LLL_MUTEX_LOCK (mutex);
 
 	  mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
 	}
-- 
2.7.4