From 8162147872491bb5b48e91543b19c49a29ae6b6d Mon Sep 17 00:00:00 2001
From: Wangyang Guo <wangyang.guo@intel.com>
Date: Fri, 6 May 2022 01:50:10 +0000
Subject: [PATCH] nptl: Add backoff mechanism to spinlock loop
Content-type: text/plain; charset=UTF-8

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.

v3:
1. Restore read-check since it works well in some platform.
2. Make backoff arch dependent, and enable it for x86_64.
3. Limit max backoff to reduce latency in large critical section.

v4: Fix strict-prototypes error in sysdeps/nptl/pthread_mutex_backoff.h

v5: Commit log updated for regression in large critical section.

Result of pthread-mutex-locks bench

Test Platform: Xeon 8280L (2 socket, 112 CPUs in total)
First Row: thread number
First Col: critical section length
Values: backoff vs upstream, time based, low is better

non-critical-length: 1
	1	2	4	8	16	32	64	112	140
0	0.99	0.58	0.52	0.49	0.43	0.44	0.46	0.52	0.54
1	0.98	0.43	0.56	0.50	0.44	0.45	0.50	0.56	0.57
2	0.99	0.41	0.57	0.51	0.45	0.47	0.48	0.60	0.61
4	0.99	0.45	0.59	0.53	0.48	0.49	0.52	0.64	0.65
8	1.00	0.66	0.71	0.63	0.56	0.59	0.66	0.72	0.71
16	0.97	0.78	0.91	0.73	0.67	0.70	0.79	0.80	0.80
32	0.95	1.17	0.98	0.87	0.82	0.86	0.89	0.90	0.90
64	0.96	0.95	1.01	1.01	0.98	1.00	1.03	0.99	0.99
128	0.99	1.01	1.01	1.17	1.08	1.12	1.02	0.97	1.02

non-critical-length: 32
	1	2	4	8	16	32	64	112	140
0	1.03	0.97	0.75	0.65	0.58	0.58	0.56	0.70	0.70
1	0.94	0.95	0.76	0.65	0.58	0.58	0.61	0.71	0.72
2	0.97	0.96	0.77	0.66	0.58	0.59	0.62	0.74	0.74
4	0.99	0.96	0.78	0.66	0.60	0.61	0.66	0.76	0.77
8	0.99	0.99	0.84	0.70	0.64	0.66	0.71	0.80	0.80
16	0.98	0.97	0.95	0.76	0.70	0.73	0.81	0.85	0.84
32	1.04	1.12	1.04	0.89	0.82	0.86	0.93	0.91	0.91
64	0.99	1.15	1.07	1.00	0.99	1.01	1.05	0.99	0.99
128	1.00	1.21	1.20	1.22	1.25	1.31	1.12	1.10	0.99

non-critical-length: 128
	1	2	4	8	16	32	64	112	140
0	1.02	1.00	0.99	0.67	0.61	0.61	0.61	0.74	0.73
1	0.95	0.99	1.00	0.68	0.61	0.60	0.60	0.74	0.74
2	1.00	1.04	1.00	0.68	0.59	0.61	0.65	0.76	0.76
4	1.00	0.96	0.98	0.70	0.63	0.63	0.67	0.78	0.77
8	1.01	1.02	0.89	0.73	0.65	0.67	0.71	0.81	0.80
16	0.99	0.96	0.96	0.79	0.71	0.73	0.80	0.84	0.84
32	0.99	0.95	1.05	0.89	0.84	0.85	0.94	0.92	0.91
64	1.00	0.99	1.16	1.04	1.00	1.02	1.06	0.99	0.99
128	1.00	1.06	0.98	1.14	1.39	1.26	1.08	1.02	0.98

There is regression in large critical section. But adaptive mutex is
aimed for "quick" locks. Small critical section is more common when
users choose to use adaptive pthread_mutex.

Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Conflicts:
	pthreadP.h
	(had been moved)
	nptl/pthread_mutex_lock.c
	(max_adaptive_count renamed)

---
 nptl/pthreadP.h                             |  1 +
 nptl/pthread_mutex_lock.c                   | 16 +++++++--
 sysdeps/nptl/pthread_mutex_backoff.h        | 35 ++++++++++++++++++
 sysdeps/x86_64/nptl/pthread_mutex_backoff.h | 39 +++++++++++++++++++++
 4 files changed, 89 insertions(+), 2 deletions(-)
 create mode 100644 sysdeps/nptl/pthread_mutex_backoff.h
 create mode 100644 sysdeps/x86_64/nptl/pthread_mutex_backoff.h

diff --git a/nptl/pthreadP.h b/nptl/pthreadP.h
index 7ddc166c..1550e3b6 100644
--- a/nptl/pthreadP.h
+++ b/nptl/pthreadP.h
@@ -33,6 +33,7 @@
 #include <kernel-features.h>
 #include <errno.h>
 #include <internal-signals.h>
+#include <pthread_mutex_backoff.h>
 
 
 /* Atomic operations on TLS memory.  */
diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index d96a9933..c7770fc9 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -133,14 +133,26 @@ __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 = get_jitter ();
 	  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);
+	      /* Prepare for next loop.  */
+	      exp_backoff = get_next_backoff (exp_backoff);
 	    }
 	  while (LLL_MUTEX_READ_LOCK (mutex) != 0
 		 || LLL_MUTEX_TRYLOCK (mutex) != 0);
diff --git a/sysdeps/nptl/pthread_mutex_backoff.h b/sysdeps/nptl/pthread_mutex_backoff.h
new file mode 100644
index 00000000..5b26c22a
--- /dev/null
+++ b/sysdeps/nptl/pthread_mutex_backoff.h
@@ -0,0 +1,35 @@
+/* Pthread mutex backoff configuration.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+#ifndef _PTHREAD_MUTEX_BACKOFF_H
+#define _PTHREAD_MUTEX_BACKOFF_H 1
+
+static inline unsigned int
+get_jitter (void)
+{
+  /* Arch dependent random jitter, return 0 disables random.  */
+  return 0;
+}
+
+static inline int
+get_next_backoff (int backoff)
+{
+  /* Next backoff, return 1 disables mutex backoff.  */
+  return 1;
+}
+
+#endif
diff --git a/sysdeps/x86_64/nptl/pthread_mutex_backoff.h b/sysdeps/x86_64/nptl/pthread_mutex_backoff.h
new file mode 100644
index 00000000..ec74c3d9
--- /dev/null
+++ b/sysdeps/x86_64/nptl/pthread_mutex_backoff.h
@@ -0,0 +1,39 @@
+/* Pthread mutex backoff configuration.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+#ifndef _PTHREAD_MUTEX_BACKOFF_H
+#define _PTHREAD_MUTEX_BACKOFF_H 1
+
+#include <fast-jitter.h>
+
+static inline unsigned int
+get_jitter (void)
+{
+  return get_fast_jitter ();
+}
+
+#define MAX_BACKOFF 16
+
+static inline int
+get_next_backoff (int backoff)
+{
+  /* Binary expontial backoff. Limiting max backoff
+     can reduce latency in large critical section.  */
+  return (backoff < MAX_BACKOFF) ? backoff << 1 : backoff;
+}
+
+#endif
-- 
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