nptl: Add backoff mechanism to spinlock loop

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>

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index d2e652d151d7ba3d8c9cb5f1b5a3ce8d2720881e..6e767a87247063c0ac84242ef13e72af79021104 100644 (file)
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -138,14 +138,26 @@ 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/pthreadP.h b/sysdeps/nptl/pthreadP.h
index 601db4ff2b4880e873adaf312039cec628466217..39af275c254ef3e737736bd5c38099bada8746d6 100644 (file)
--- a/sysdeps/nptl/pthreadP.h
+++ b/sysdeps/nptl/pthreadP.h
@@ -33,6 +33,7 @@
 #include <kernel-features.h>
 #include <errno.h>
 #include <internal-signals.h>
+#include <pthread_mutex_backoff.h>
 #include "pthread_mutex_conf.h"
 
 

diff --git a/sysdeps/nptl/pthread_mutex_backoff.h b/sysdeps/nptl/pthread_mutex_backoff.h
new file mode 100644 (file)
index 0000000..5b26c22
--- /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 (file)
index 0000000..ec74c3d
--- /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