[PATCH v2] nptl: Add backoff mechanism to spinlock loop

[Wangyang Guo]

When mutiple threads waiting for lock at the same time, once lock owner
releases the lock, waiters will see lock available and all try to lock,
which may cause an expensive CAS storm.

Binary exponential backoff with random jitter is introduced. As try-lock
attempt increases, there is more likely that a larger number threads
compete for adaptive mutex lock, so increase wait time in exponential.
A random jitter is also added to avoid synchronous try-lock from other
threads.

v2: Remove read-check before try-lock for performance.

Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
---
 nptl/pthread_mutex_lock.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index d2e652d151..7e75ec1cba 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -26,6 +26,7 @@
 #include <futex-internal.h>
 #include <stap-probe.h>
 #include <shlib-compat.h>
+#include <random-bits.h>
 
 /* Some of the following definitions differ when pthread_mutex_cond_lock.c
    includes this file.  */
@@ -64,11 +65,6 @@ lll_mutex_lock_optimized (pthread_mutex_t *mutex)
 # define PTHREAD_MUTEX_VERSIONS 1
 #endif
 
-#ifndef LLL_MUTEX_READ_LOCK
-# define LLL_MUTEX_READ_LOCK(mutex) \
-  atomic_load_relaxed (&(mutex)->__data.__lock)
-#endif
-
 static int __pthread_mutex_lock_full (pthread_mutex_t *mutex)
      __attribute_noinline__;
 
@@ -138,17 +134,28 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
 	  int cnt = 0;
 	  int max_cnt = MIN (max_adaptive_count (),
 			     mutex->__data.__spins * 2 + 10);
+	  int spin_count, exp_backoff = 1;
+	  unsigned int jitter = random_bits ();
 	  do
 	    {
-	      if (cnt++ >= max_cnt)
+	      /* In each loop, spin count is exponential backoff plus
+	         random jitter, random range is [0, exp_backoff-1].  */
+	      spin_count = exp_backoff + (jitter & (exp_backoff - 1));
+	      cnt += spin_count;
+	      if (cnt >= max_cnt)
 		{
+		  /* If cnt exceeds max spin count, just go to wait
+		     queue.  */
 		  LLL_MUTEX_LOCK (mutex);
 		  break;
 		}
-	      atomic_spin_nop ();
+	      do
+		  atomic_spin_nop ();
+	      while (--spin_count > 0);
+	      /* Binary exponential backoff, prepare for next loop.  */
+	      exp_backoff <<= 1;
 	    }
-	  while (LLL_MUTEX_READ_LOCK (mutex) != 0
-		 || LLL_MUTEX_TRYLOCK (mutex) != 0);
+	  while (LLL_MUTEX_TRYLOCK (mutex) != 0);
 
 	  mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
 	}
-- 
2.35.1