1 /* elision-conf.c: Lock elision tunable parameters.
2 Copyright (C) 2015-2021 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 <elision-conf.h>
22 #include <unistd.h>
23 #include <dl-procinfo.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
33 struct elision_config __elision_aconf =
34 {
35 /* How many times to use a non-transactional lock after a transactional
36 failure has occurred because the lock is already acquired. Expressed
37 in number of lock 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 to not attempt to use elision if a lock used up all retries
44 without success. Expressed in number of lock acquisition attempts. */
45 .skip_lock_out_of_tbegin_retries = 3,
46 /* How often we retry using elision if there is chance for the transaction
47 to finish execution (e.g., it wasn't aborted due to the lock being
48 already acquired. */
49 .try_tbegin = 3,
50 /* Same as SKIP_LOCK_INTERNAL_ABORT but for trylock. */
51 .skip_trylock_internal_abort = 3,
52 };
53
54 #if HAVE_TUNABLES
55 static inline void
56 __always_inline
do_set_elision_enable(int32_t elision_enable)57 do_set_elision_enable (int32_t elision_enable)
58 {
59 /* Enable elision if it's avaliable in hardware. It's not necessary to check
60 if __libc_enable_secure isn't enabled since elision_enable will be set
61 according to the default, which is disabled. */
62 if (elision_enable == 1)
63 __pthread_force_elision = (GLRO (dl_hwcap2)
64 & PPC_FEATURE2_HAS_HTM) ? 1 : 0;
65 }
66
67 /* The pthread->elision_enable tunable is 0 or 1 indicating that elision
68 should be disabled or enabled respectively. The feature will only be used
69 if it's supported by the hardware. */
70
71 void
TUNABLE_CALLBACK(set_elision_enable)72 TUNABLE_CALLBACK (set_elision_enable) (tunable_val_t *valp)
73 {
74 int32_t elision_enable = (int32_t) valp->numval;
75 do_set_elision_enable (elision_enable);
76 }
77
78 #define TUNABLE_CALLBACK_FNDECL(__name, __type) \
79 static inline void \
80 __always_inline \
81 do_set_elision_ ## __name (__type value) \
82 { \
83 __elision_aconf.__name = value; \
84 } \
85 void \
86 TUNABLE_CALLBACK (set_elision_ ## __name) (tunable_val_t *valp) \
87 { \
88 __type value = (__type) (valp)->numval; \
89 do_set_elision_ ## __name (value); \
90 }
91
92 TUNABLE_CALLBACK_FNDECL (skip_lock_busy, int32_t);
93 TUNABLE_CALLBACK_FNDECL (skip_lock_internal_abort, int32_t);
94 TUNABLE_CALLBACK_FNDECL (skip_lock_out_of_tbegin_retries, int32_t);
95 TUNABLE_CALLBACK_FNDECL (try_tbegin, int32_t);
96 TUNABLE_CALLBACK_FNDECL (skip_trylock_internal_abort, int32_t);
97 #endif
98
99 /* Initialize elision. */
100
101 void
__lll_elision_init(void)102 __lll_elision_init (void)
103 {
104 #if HAVE_TUNABLES
105 /* Elision depends on tunables and must be explicitly turned on by setting
106 the appropriate tunable on a supported platform. */
107
108 TUNABLE_GET (enable, int32_t,
109 TUNABLE_CALLBACK (set_elision_enable));
110 TUNABLE_GET (skip_lock_busy, int32_t,
111 TUNABLE_CALLBACK (set_elision_skip_lock_busy));
112 TUNABLE_GET (skip_lock_internal_abort, int32_t,
113 TUNABLE_CALLBACK (set_elision_skip_lock_internal_abort));
114 TUNABLE_GET (skip_lock_after_retries, int32_t,
115 TUNABLE_CALLBACK (set_elision_skip_lock_out_of_tbegin_retries));
116 TUNABLE_GET (tries, int32_t,
117 TUNABLE_CALLBACK (set_elision_try_tbegin));
118 TUNABLE_GET (skip_trylock_internal_abort, int32_t,
119 TUNABLE_CALLBACK (set_elision_skip_trylock_internal_abort));
120 #endif
121
122 /* Linux from 3.9 through 4.2 do not abort HTM transaction on syscalls,
123 instead it suspends the transaction and resumes it when returning to
124 usercode. The side-effects of the syscall will always remain visible,
125 even if the transaction is aborted. This is an issue when a transaction
126 is used along with futex syscall, on pthread_cond_wait for instance,
127 where futex might succeed but the transaction is rolled back leading
128 the condition variable object in an inconsistent state.
129
130 Glibc used to prevent it by always aborting a transaction before issuing
131 a syscall. Linux 4.2 also decided to abort active transaction in
132 syscalls which makes the glibc workaround superflours. Worse, glibc
133 transaction abortions leads to a performance issues on recent kernels.
134
135 So Lock Elision is just enabled when it has been explict set (either
136 by tunables of by a configure switch) and if kernel aborts HTM
137 transactions on syscalls (PPC_FEATURE2_HTM_NOSC) */
138
139 __pthread_force_elision = (__pthread_force_elision
140 && GLRO (dl_hwcap2) & PPC_FEATURE2_HTM_NOSC);
141
142 if (!__pthread_force_elision)
143 __elision_aconf.try_tbegin = 0; /* Disable elision on rwlocks. */
144 }
145