1/* elision-conf.c: Lock elision tunable parameters.
2 Copyright (C) 2013-2023 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include "config.h"
20#include <pthreadP.h>
21#include <init-arch.h>
22#include <elision-conf.h>
23#include <unistd.h>
24
25#if HAVE_TUNABLES
26# define TUNABLE_NAMESPACE elision
27#endif
28#include <elf/dl-tunables.h>
29
30/* Reasonable initial tuning values, may be revised in the future.
31 This is a conservative initial value. */
32
33struct elision_config __elision_aconf =
34 {
35 /* How often to not attempt to use elision if a transaction aborted
36 because the lock is already acquired. Expressed in number of lock
37 acquisition attempts. */
38 .skip_lock_busy = 3,
39 /* How often to not attempt to use elision if a transaction aborted due
40 to reasons other than other threads' memory accesses. Expressed in
41 number of lock acquisition attempts. */
42 .skip_lock_internal_abort = 3,
43 /* How often we retry using elision if there is chance for the transaction
44 to finish execution (e.g., it wasn't aborted due to the lock being
45 already acquired. */
46 .retry_try_xbegin = 3,
47 /* Same as SKIP_LOCK_INTERNAL_ABORT but for trylock. */
48 .skip_trylock_internal_abort = 3,
49 };
50
51#if HAVE_TUNABLES
52static __always_inline void
53do_set_elision_enable (int32_t elision_enable)
54{
55 /* Enable elision if it's avaliable in hardware. It's not necessary to check
56 if __libc_enable_secure isn't enabled since elision_enable will be set
57 according to the default, which is disabled. */
58 if (elision_enable == 1)
59 __pthread_force_elision = CPU_FEATURE_USABLE (RTM) ? 1 : 0;
60}
61
62/* The pthread->elision_enable tunable is 0 or 1 indicating that elision
63 should be disabled or enabled respectively. The feature will only be used
64 if it's supported by the hardware. */
65
66void
67TUNABLE_CALLBACK (set_elision_enable) (tunable_val_t *valp)
68{
69 int32_t elision_enable = (int32_t) valp->numval;
70 do_set_elision_enable (elision_enable);
71}
72
73#define TUNABLE_CALLBACK_FNDECL(__name, __type) \
74static __always_inline void \
75do_set_elision_ ## __name (__type value) \
76{ \
77 __elision_aconf.__name = value; \
78} \
79void \
80TUNABLE_CALLBACK (set_elision_ ## __name) (tunable_val_t *valp) \
81{ \
82 __type value = (__type) (valp)->numval; \
83 do_set_elision_ ## __name (value); \
84}
85
86TUNABLE_CALLBACK_FNDECL (skip_lock_busy, int32_t);
87TUNABLE_CALLBACK_FNDECL (skip_lock_internal_abort, int32_t);
88TUNABLE_CALLBACK_FNDECL (retry_try_xbegin, int32_t);
89TUNABLE_CALLBACK_FNDECL (skip_trylock_internal_abort, int32_t);
90#endif
91
92/* Initialize elision. */
93
94void
95__lll_elision_init (void)
96{
97#if HAVE_TUNABLES
98 /* Elision depends on tunables and must be explicitly turned on by setting
99 the appropriate tunable on a supported platform. */
100
101 TUNABLE_GET (enable, int32_t,
102 TUNABLE_CALLBACK (set_elision_enable));
103 TUNABLE_GET (skip_lock_busy, int32_t,
104 TUNABLE_CALLBACK (set_elision_skip_lock_busy));
105 TUNABLE_GET (skip_lock_internal_abort, int32_t,
106 TUNABLE_CALLBACK (set_elision_skip_lock_internal_abort));
107 TUNABLE_GET (tries, int32_t,
108 TUNABLE_CALLBACK (set_elision_retry_try_xbegin));
109 TUNABLE_GET (skip_trylock_internal_abort, int32_t,
110 TUNABLE_CALLBACK (set_elision_skip_trylock_internal_abort));
111#endif
112
113 if (!__pthread_force_elision)
114 __elision_aconf.retry_try_xbegin = 0; /* Disable elision on rwlocks. */
115}
116