sysdeps/unix/sysv/linux/x86/elision-conf.c

   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 #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 often to not attempt to use elision if a transaction aborted
  34        because the lock is already acquired.  Expressed in number of lock
  35        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 we retry using elision if there is chance for the transaction
  42        to finish execution (e.g., it wasn't aborted due to the lock being
  43        already acquired.  */
  44     .retry_try_xbegin = 3,
  45     /* Same as SKIP_LOCK_INTERNAL_ABORT but for trylock.  */
  46     .skip_trylock_internal_abort = 3,
  47   };
  48
  49 static __always_inline void
  50 do_set_elision_enable (int32_t elision_enable)
  51 {
  52   /* Enable elision if it's avaliable in hardware. It's not necessary to check
  53      if __libc_enable_secure isn't enabled since elision_enable will be set
  54      according to the default, which is disabled.  */
  55   if (elision_enable == 1)
  56     __pthread_force_elision = CPU_FEATURE_USABLE (RTM) ? 1 : 0;
  57 }
  58
  59 /* The pthread->elision_enable tunable is 0 or 1 indicating that elision
  60    should be disabled or enabled respectively.  The feature will only be used
  61    if it's supported by the hardware.  */
  62
  63 void
  64 TUNABLE_CALLBACK (set_elision_enable) (tunable_val_t *valp)
  65 {
  66   int32_t elision_enable = (int32_t) valp->numval;
  67   do_set_elision_enable (elision_enable);
  68 }
  69
  70 #define TUNABLE_CALLBACK_FNDECL(__name, __type)                 \
  71 static __always_inline void                                     \
  72 do_set_elision_ ## __name (__type value)                        \
  73 {                                                               \
  74   __elision_aconf.__name = value;                               \
  75 }                                                               \
  76 void                                                            \
  77 TUNABLE_CALLBACK (set_elision_ ## __name) (tunable_val_t *valp) \
  78 {                                                               \
  79   __type value = (__type) (valp)->numval;                       \
  80   do_set_elision_ ## __name (value);                            \
  81 }
  82
  83 TUNABLE_CALLBACK_FNDECL (skip_lock_busy, int32_t);
  84 TUNABLE_CALLBACK_FNDECL (skip_lock_internal_abort, int32_t);
  85 TUNABLE_CALLBACK_FNDECL (retry_try_xbegin, int32_t);
  86 TUNABLE_CALLBACK_FNDECL (skip_trylock_internal_abort, int32_t);
  87
  88 /* Initialize elision.  */
  89
  90 void
  91 __lll_elision_init (void)
  92 {
  93   /* Elision depends on tunables and must be explicitly turned on by setting
  94      the appropriate tunable on a supported platform.  */
  95
  96   TUNABLE_GET (enable, int32_t,
  97                TUNABLE_CALLBACK (set_elision_enable));
  98   TUNABLE_GET (skip_lock_busy, int32_t,
  99                TUNABLE_CALLBACK (set_elision_skip_lock_busy));
 100   TUNABLE_GET (skip_lock_internal_abort, int32_t,
 101                TUNABLE_CALLBACK (set_elision_skip_lock_internal_abort));
 102   TUNABLE_GET (tries, int32_t,
 103                TUNABLE_CALLBACK (set_elision_retry_try_xbegin));
 104   TUNABLE_GET (skip_trylock_internal_abort, int32_t,
 105                TUNABLE_CALLBACK (set_elision_skip_trylock_internal_abort));
 106
 107   if (!__pthread_force_elision)
 108     __elision_aconf.retry_try_xbegin = 0; /* Disable elision on rwlocks.  */
 109 }