1 | /* elision-conf.c: Lock elision tunable parameters. |
2 | Copyright (C) 2015-2022 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 |
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 |
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 |
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 | |