210 lines
8.5 KiB
Diff
210 lines
8.5 KiB
Diff
commit ea69248445fb9b80da02ee0c7261cba4b1a5532e
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Author: Wangyang Guo <wangyang.guo@intel.com>
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Date: Fri May 6 01:50:10 2022 +0000
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nptl: Add backoff mechanism to spinlock loop
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When mutiple threads waiting for lock at the same time, once lock owner
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releases the lock, waiters will see lock available and all try to lock,
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which may cause an expensive CAS storm.
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Binary exponential backoff with random jitter is introduced. As try-lock
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attempt increases, there is more likely that a larger number threads
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compete for adaptive mutex lock, so increase wait time in exponential.
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A random jitter is also added to avoid synchronous try-lock from other
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threads.
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v2: Remove read-check before try-lock for performance.
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v3:
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1. Restore read-check since it works well in some platform.
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2. Make backoff arch dependent, and enable it for x86_64.
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3. Limit max backoff to reduce latency in large critical section.
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v4: Fix strict-prototypes error in sysdeps/nptl/pthread_mutex_backoff.h
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v5: Commit log updated for regression in large critical section.
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Result of pthread-mutex-locks bench
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Test Platform: Xeon 8280L (2 socket, 112 CPUs in total)
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First Row: thread number
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First Col: critical section length
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Values: backoff vs upstream, time based, low is better
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non-critical-length: 1
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1 2 4 8 16 32 64 112 140
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0 0.99 0.58 0.52 0.49 0.43 0.44 0.46 0.52 0.54
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1 0.98 0.43 0.56 0.50 0.44 0.45 0.50 0.56 0.57
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2 0.99 0.41 0.57 0.51 0.45 0.47 0.48 0.60 0.61
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4 0.99 0.45 0.59 0.53 0.48 0.49 0.52 0.64 0.65
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8 1.00 0.66 0.71 0.63 0.56 0.59 0.66 0.72 0.71
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16 0.97 0.78 0.91 0.73 0.67 0.70 0.79 0.80 0.80
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32 0.95 1.17 0.98 0.87 0.82 0.86 0.89 0.90 0.90
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64 0.96 0.95 1.01 1.01 0.98 1.00 1.03 0.99 0.99
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128 0.99 1.01 1.01 1.17 1.08 1.12 1.02 0.97 1.02
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non-critical-length: 32
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1 2 4 8 16 32 64 112 140
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0 1.03 0.97 0.75 0.65 0.58 0.58 0.56 0.70 0.70
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1 0.94 0.95 0.76 0.65 0.58 0.58 0.61 0.71 0.72
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2 0.97 0.96 0.77 0.66 0.58 0.59 0.62 0.74 0.74
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4 0.99 0.96 0.78 0.66 0.60 0.61 0.66 0.76 0.77
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8 0.99 0.99 0.84 0.70 0.64 0.66 0.71 0.80 0.80
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16 0.98 0.97 0.95 0.76 0.70 0.73 0.81 0.85 0.84
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32 1.04 1.12 1.04 0.89 0.82 0.86 0.93 0.91 0.91
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64 0.99 1.15 1.07 1.00 0.99 1.01 1.05 0.99 0.99
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128 1.00 1.21 1.20 1.22 1.25 1.31 1.12 1.10 0.99
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non-critical-length: 128
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1 2 4 8 16 32 64 112 140
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0 1.02 1.00 0.99 0.67 0.61 0.61 0.61 0.74 0.73
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1 0.95 0.99 1.00 0.68 0.61 0.60 0.60 0.74 0.74
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2 1.00 1.04 1.00 0.68 0.59 0.61 0.65 0.76 0.76
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4 1.00 0.96 0.98 0.70 0.63 0.63 0.67 0.78 0.77
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8 1.01 1.02 0.89 0.73 0.65 0.67 0.71 0.81 0.80
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16 0.99 0.96 0.96 0.79 0.71 0.73 0.80 0.84 0.84
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32 0.99 0.95 1.05 0.89 0.84 0.85 0.94 0.92 0.91
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64 1.00 0.99 1.16 1.04 1.00 1.02 1.06 0.99 0.99
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128 1.00 1.06 0.98 1.14 1.39 1.26 1.08 1.02 0.98
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There is regression in large critical section. But adaptive mutex is
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aimed for "quick" locks. Small critical section is more common when
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users choose to use adaptive pthread_mutex.
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Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
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Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
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(cherry picked from commit 8162147872491bb5b48e91543b19c49a29ae6b6d)
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diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
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index bbe754a272b97d91..8f3f687f2a151d16 100644
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--- a/nptl/pthread_mutex_lock.c
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+++ b/nptl/pthread_mutex_lock.c
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@@ -139,14 +139,26 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
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int cnt = 0;
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int max_cnt = MIN (max_adaptive_count (),
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mutex->__data.__spins * 2 + 10);
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+ int spin_count, exp_backoff = 1;
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+ unsigned int jitter = get_jitter ();
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do
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{
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- if (cnt++ >= max_cnt)
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+ /* In each loop, spin count is exponential backoff plus
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+ random jitter, random range is [0, exp_backoff-1]. */
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+ spin_count = exp_backoff + (jitter & (exp_backoff - 1));
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+ cnt += spin_count;
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+ if (cnt >= max_cnt)
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{
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+ /* If cnt exceeds max spin count, just go to wait
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+ queue. */
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LLL_MUTEX_LOCK (mutex);
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break;
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}
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- atomic_spin_nop ();
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+ do
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+ atomic_spin_nop ();
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+ while (--spin_count > 0);
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+ /* Prepare for next loop. */
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+ exp_backoff = get_next_backoff (exp_backoff);
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}
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while (LLL_MUTEX_READ_LOCK (mutex) != 0
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|| LLL_MUTEX_TRYLOCK (mutex) != 0);
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diff --git a/sysdeps/nptl/pthreadP.h b/sysdeps/nptl/pthreadP.h
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index b968afc4c6b61b92..ed186ce3df1fde0c 100644
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--- a/sysdeps/nptl/pthreadP.h
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+++ b/sysdeps/nptl/pthreadP.h
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@@ -34,6 +34,7 @@
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#include <kernel-features.h>
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#include <errno.h>
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#include <internal-signals.h>
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+#include <pthread_mutex_backoff.h>
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#include "pthread_mutex_conf.h"
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diff --git a/sysdeps/nptl/pthread_mutex_backoff.h b/sysdeps/nptl/pthread_mutex_backoff.h
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new file mode 100644
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index 0000000000000000..5b26c22ac789f54f
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--- /dev/null
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+++ b/sysdeps/nptl/pthread_mutex_backoff.h
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@@ -0,0 +1,35 @@
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+/* Pthread mutex backoff configuration.
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+ Copyright (C) 2022 Free Software Foundation, Inc.
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+ This file is part of the GNU C Library.
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+
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+ The GNU C Library is free software; you can redistribute it and/or
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+ modify it under the terms of the GNU Lesser General Public
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+ License as published by the Free Software Foundation; either
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+ version 2.1 of the License, or (at your option) any later version.
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+
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+ The GNU C Library is distributed in the hope that it will be useful,
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+ but WITHOUT ANY WARRANTY; without even the implied warranty of
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+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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+ Lesser General Public License for more details.
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+
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+ You should have received a copy of the GNU Lesser General Public
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+ License along with the GNU C Library; if not, see
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+ <https://www.gnu.org/licenses/>. */
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+#ifndef _PTHREAD_MUTEX_BACKOFF_H
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+#define _PTHREAD_MUTEX_BACKOFF_H 1
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+
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+static inline unsigned int
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+get_jitter (void)
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+{
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+ /* Arch dependent random jitter, return 0 disables random. */
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+ return 0;
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+}
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+
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+static inline int
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+get_next_backoff (int backoff)
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+{
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+ /* Next backoff, return 1 disables mutex backoff. */
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+ return 1;
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+}
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+
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+#endif
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diff --git a/sysdeps/x86_64/nptl/pthread_mutex_backoff.h b/sysdeps/x86_64/nptl/pthread_mutex_backoff.h
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new file mode 100644
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index 0000000000000000..ec74c3d9db61864e
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--- /dev/null
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+++ b/sysdeps/x86_64/nptl/pthread_mutex_backoff.h
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@@ -0,0 +1,39 @@
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+/* Pthread mutex backoff configuration.
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+ Copyright (C) 2022 Free Software Foundation, Inc.
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+ This file is part of the GNU C Library.
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+
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+ The GNU C Library is free software; you can redistribute it and/or
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+ modify it under the terms of the GNU Lesser General Public
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+ License as published by the Free Software Foundation; either
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+ version 2.1 of the License, or (at your option) any later version.
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+
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+ The GNU C Library is distributed in the hope that it will be useful,
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+ but WITHOUT ANY WARRANTY; without even the implied warranty of
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+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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+ Lesser General Public License for more details.
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+
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+ You should have received a copy of the GNU Lesser General Public
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+ License along with the GNU C Library; if not, see
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+ <https://www.gnu.org/licenses/>. */
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+#ifndef _PTHREAD_MUTEX_BACKOFF_H
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+#define _PTHREAD_MUTEX_BACKOFF_H 1
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+
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+#include <fast-jitter.h>
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+
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+static inline unsigned int
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+get_jitter (void)
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+{
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+ return get_fast_jitter ();
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+}
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+
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+#define MAX_BACKOFF 16
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+
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+static inline int
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+get_next_backoff (int backoff)
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+{
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+ /* Binary expontial backoff. Limiting max backoff
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+ can reduce latency in large critical section. */
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+ return (backoff < MAX_BACKOFF) ? backoff << 1 : backoff;
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+}
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+
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+#endif
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