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The elision interfaces are closely aligned between the targets that implement them, so declare them in the generic <lowlevellock.h> file. Empty .c stubs are provided, so that fewer makefile updates under sysdeps are needed. Also simplify initialization via __libc_early_init. The symbols __lll_clocklock_elision, __lll_lock_elision, __lll_trylock_elision, __lll_unlock_elision, __pthread_force_elision move into libc. For the time being, non-hidden references are used from libpthread to access them, but once that part of libpthread is moved into libc, hidden symbols will be used again. (Hidden references seem desirable to reduce the likelihood of transactions aborts.)
145 lines
5.6 KiB
C
145 lines
5.6 KiB
C
/* elision-conf.c: Lock elision tunable parameters.
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Copyright (C) 2015-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include "config.h"
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#include <pthreadP.h>
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#include <elision-conf.h>
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#include <unistd.h>
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#include <dl-procinfo.h>
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#if HAVE_TUNABLES
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# define TUNABLE_NAMESPACE elision
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#endif
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#include <elf/dl-tunables.h>
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/* Reasonable initial tuning values, may be revised in the future.
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This is a conservative initial value. */
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struct elision_config __elision_aconf =
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{
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/* How many times to use a non-transactional lock after a transactional
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failure has occurred because the lock is already acquired. Expressed
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in number of lock acquisition attempts. */
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.skip_lock_busy = 3,
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/* How often to not attempt to use elision if a transaction aborted due
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to reasons other than other threads' memory accesses. Expressed in
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number of lock acquisition attempts. */
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.skip_lock_internal_abort = 3,
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/* How often to not attempt to use elision if a lock used up all retries
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without success. Expressed in number of lock acquisition attempts. */
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.skip_lock_out_of_tbegin_retries = 3,
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/* How often we retry using elision if there is chance for the transaction
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to finish execution (e.g., it wasn't aborted due to the lock being
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already acquired. */
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.try_tbegin = 3,
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/* Same as SKIP_LOCK_INTERNAL_ABORT but for trylock. */
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.skip_trylock_internal_abort = 3,
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};
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#if HAVE_TUNABLES
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static inline void
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__always_inline
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do_set_elision_enable (int32_t elision_enable)
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{
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/* Enable elision if it's avaliable in hardware. It's not necessary to check
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if __libc_enable_secure isn't enabled since elision_enable will be set
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according to the default, which is disabled. */
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if (elision_enable == 1)
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__pthread_force_elision = (GLRO (dl_hwcap2)
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& PPC_FEATURE2_HAS_HTM) ? 1 : 0;
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}
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/* The pthread->elision_enable tunable is 0 or 1 indicating that elision
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should be disabled or enabled respectively. The feature will only be used
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if it's supported by the hardware. */
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void
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TUNABLE_CALLBACK (set_elision_enable) (tunable_val_t *valp)
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{
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int32_t elision_enable = (int32_t) valp->numval;
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do_set_elision_enable (elision_enable);
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}
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#define TUNABLE_CALLBACK_FNDECL(__name, __type) \
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static inline void \
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__always_inline \
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do_set_elision_ ## __name (__type value) \
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{ \
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__elision_aconf.__name = value; \
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} \
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void \
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TUNABLE_CALLBACK (set_elision_ ## __name) (tunable_val_t *valp) \
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{ \
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__type value = (__type) (valp)->numval; \
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do_set_elision_ ## __name (value); \
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}
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TUNABLE_CALLBACK_FNDECL (skip_lock_busy, int32_t);
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TUNABLE_CALLBACK_FNDECL (skip_lock_internal_abort, int32_t);
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TUNABLE_CALLBACK_FNDECL (skip_lock_out_of_tbegin_retries, int32_t);
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TUNABLE_CALLBACK_FNDECL (try_tbegin, int32_t);
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TUNABLE_CALLBACK_FNDECL (skip_trylock_internal_abort, int32_t);
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#endif
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/* Initialize elision. */
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void
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__lll_elision_init (void)
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{
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#if HAVE_TUNABLES
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/* Elision depends on tunables and must be explicitly turned on by setting
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the appropriate tunable on a supported platform. */
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TUNABLE_GET (enable, int32_t,
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TUNABLE_CALLBACK (set_elision_enable));
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TUNABLE_GET (skip_lock_busy, int32_t,
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TUNABLE_CALLBACK (set_elision_skip_lock_busy));
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TUNABLE_GET (skip_lock_internal_abort, int32_t,
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TUNABLE_CALLBACK (set_elision_skip_lock_internal_abort));
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TUNABLE_GET (skip_lock_after_retries, int32_t,
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TUNABLE_CALLBACK (set_elision_skip_lock_out_of_tbegin_retries));
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TUNABLE_GET (tries, int32_t,
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TUNABLE_CALLBACK (set_elision_try_tbegin));
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TUNABLE_GET (skip_trylock_internal_abort, int32_t,
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TUNABLE_CALLBACK (set_elision_skip_trylock_internal_abort));
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#endif
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/* Linux from 3.9 through 4.2 do not abort HTM transaction on syscalls,
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instead it suspends the transaction and resumes it when returning to
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usercode. The side-effects of the syscall will always remain visible,
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even if the transaction is aborted. This is an issue when a transaction
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is used along with futex syscall, on pthread_cond_wait for instance,
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where futex might succeed but the transaction is rolled back leading
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the condition variable object in an inconsistent state.
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Glibc used to prevent it by always aborting a transaction before issuing
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a syscall. Linux 4.2 also decided to abort active transaction in
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syscalls which makes the glibc workaround superflours. Worse, glibc
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transaction abortions leads to a performance issues on recent kernels.
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So Lock Elision is just enabled when it has been explict set (either
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by tunables of by a configure switch) and if kernel aborts HTM
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transactions on syscalls (PPC_FEATURE2_HTM_NOSC) */
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__pthread_force_elision = (__pthread_force_elision
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&& GLRO (dl_hwcap2) & PPC_FEATURE2_HTM_NOSC);
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if (!__pthread_force_elision)
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__elision_aconf.try_tbegin = 0; /* Disable elision on rwlocks. */
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}
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