1 | /* elision-conf.c: Lock elision tunable parameters. |
2 | Copyright (C) 2015-2024 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 | #define TUNABLE_NAMESPACE elision |
26 | #include <elf/dl-tunables.h> |
27 | |
28 | /* Reasonable initial tuning values, may be revised in the future. |
29 | This is a conservative initial value. */ |
30 | |
31 | struct elision_config __elision_aconf = |
32 | { |
33 | /* How many times to use a non-transactional lock after a transactional |
34 | failure has occurred because the lock is already acquired. Expressed |
35 | in number of lock acquisition attempts. */ |
36 | .skip_lock_busy = 3, |
37 | /* How often to not attempt to use elision if a transaction aborted due |
38 | to reasons other than other threads' memory accesses. Expressed in |
39 | number of lock acquisition attempts. */ |
40 | .skip_lock_internal_abort = 3, |
41 | /* How often to not attempt to use elision if a lock used up all retries |
42 | without success. Expressed in number of lock acquisition attempts. */ |
43 | .skip_lock_out_of_tbegin_retries = 3, |
44 | /* How often we retry using elision if there is chance for the transaction |
45 | to finish execution (e.g., it wasn't aborted due to the lock being |
46 | already acquired. */ |
47 | .try_tbegin = 3, |
48 | /* Same as SKIP_LOCK_INTERNAL_ABORT but for trylock. */ |
49 | .skip_trylock_internal_abort = 3, |
50 | }; |
51 | |
52 | static inline void |
53 | __always_inline |
54 | do_set_elision_enable (int32_t elision_enable) |
55 | { |
56 | /* Enable elision if it's available in hardware. It's not necessary to check |
57 | if __libc_enable_secure isn't enabled since elision_enable will be set |
58 | according to the default, which is disabled. */ |
59 | if (elision_enable == 1) |
60 | __pthread_force_elision = (GLRO (dl_hwcap2) |
61 | & PPC_FEATURE2_HAS_HTM) ? 1 : 0; |
62 | } |
63 | |
64 | /* The pthread->elision_enable tunable is 0 or 1 indicating that elision |
65 | should be disabled or enabled respectively. The feature will only be used |
66 | if it's supported by the hardware. */ |
67 | |
68 | void |
69 | TUNABLE_CALLBACK (set_elision_enable) (tunable_val_t *valp) |
70 | { |
71 | int32_t elision_enable = (int32_t) valp->numval; |
72 | do_set_elision_enable (elision_enable); |
73 | } |
74 | |
75 | #define TUNABLE_CALLBACK_FNDECL(__name, __type) \ |
76 | static inline void \ |
77 | __always_inline \ |
78 | do_set_elision_ ## __name (__type value) \ |
79 | { \ |
80 | __elision_aconf.__name = value; \ |
81 | } \ |
82 | void \ |
83 | TUNABLE_CALLBACK (set_elision_ ## __name) (tunable_val_t *valp) \ |
84 | { \ |
85 | __type value = (__type) (valp)->numval; \ |
86 | do_set_elision_ ## __name (value); \ |
87 | } |
88 | |
89 | TUNABLE_CALLBACK_FNDECL (skip_lock_busy, int32_t); |
90 | TUNABLE_CALLBACK_FNDECL (skip_lock_internal_abort, int32_t); |
91 | TUNABLE_CALLBACK_FNDECL (skip_lock_out_of_tbegin_retries, int32_t); |
92 | TUNABLE_CALLBACK_FNDECL (try_tbegin, int32_t); |
93 | TUNABLE_CALLBACK_FNDECL (skip_trylock_internal_abort, int32_t); |
94 | |
95 | /* Initialize elision. */ |
96 | |
97 | void |
98 | __lll_elision_init (void) |
99 | { |
100 | /* Elision depends on tunables and must be explicitly turned on by setting |
101 | the appropriate tunable on a supported platform. */ |
102 | |
103 | TUNABLE_GET (enable, int32_t, |
104 | TUNABLE_CALLBACK (set_elision_enable)); |
105 | TUNABLE_GET (skip_lock_busy, int32_t, |
106 | TUNABLE_CALLBACK (set_elision_skip_lock_busy)); |
107 | TUNABLE_GET (skip_lock_internal_abort, int32_t, |
108 | TUNABLE_CALLBACK (set_elision_skip_lock_internal_abort)); |
109 | TUNABLE_GET (skip_lock_after_retries, int32_t, |
110 | TUNABLE_CALLBACK (set_elision_skip_lock_out_of_tbegin_retries)); |
111 | TUNABLE_GET (tries, int32_t, |
112 | TUNABLE_CALLBACK (set_elision_try_tbegin)); |
113 | TUNABLE_GET (skip_trylock_internal_abort, int32_t, |
114 | TUNABLE_CALLBACK (set_elision_skip_trylock_internal_abort)); |
115 | |
116 | /* Linux from 3.9 through 4.2 do not abort HTM transaction on syscalls, |
117 | instead it suspends the transaction and resumes it when returning to |
118 | usercode. The side-effects of the syscall will always remain visible, |
119 | even if the transaction is aborted. This is an issue when a transaction |
120 | is used along with futex syscall, on pthread_cond_wait for instance, |
121 | where futex might succeed but the transaction is rolled back leading |
122 | the condition variable object in an inconsistent state. |
123 | |
124 | Glibc used to prevent it by always aborting a transaction before issuing |
125 | a syscall. Linux 4.2 also decided to abort active transaction in |
126 | syscalls which makes the glibc workaround superflours. Worse, glibc |
127 | transaction abortions leads to a performance issues on recent kernels. |
128 | |
129 | So Lock Elision is just enabled when it has been explicitly set (either |
130 | by tunables of by a configure switch) and if kernel aborts HTM |
131 | transactions on syscalls (PPC_FEATURE2_HTM_NOSC) */ |
132 | |
133 | __pthread_force_elision = (__pthread_force_elision |
134 | && GLRO (dl_hwcap2) & PPC_FEATURE2_HTM_NOSC); |
135 | |
136 | if (!__pthread_force_elision) |
137 | __elision_aconf.try_tbegin = 0; /* Disable elision on rwlocks. */ |
138 | } |
139 | |