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/* elision-conf.c: Lock elision tunable parameters.
Copyright (C) 2015-2024 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include "config.h"
#include <pthreadP.h>
#include <elision-conf.h>
#include <unistd.h>
#include <dl-procinfo.h>
#define TUNABLE_NAMESPACE elision
#include <elf/dl-tunables.h>
/* Reasonable initial tuning values, may be revised in the future.
This is a conservative initial value. */
struct elision_config __elision_aconf =
{
/* How many times to use a non-transactional lock after a transactional
failure has occurred because the lock is already acquired. Expressed
in number of lock acquisition attempts. */
.skip_lock_busy = 3,
/* How often to not attempt to use elision if a transaction aborted due
to reasons other than other threads' memory accesses. Expressed in
number of lock acquisition attempts. */
.skip_lock_internal_abort = 3,
/* How often to not attempt to use elision if a lock used up all retries
without success. Expressed in number of lock acquisition attempts. */
.skip_lock_out_of_tbegin_retries = 3,
/* How often we retry using elision if there is chance for the transaction
to finish execution (e.g., it wasn't aborted due to the lock being
already acquired. */
.try_tbegin = 3,
/* Same as SKIP_LOCK_INTERNAL_ABORT but for trylock. */
.skip_trylock_internal_abort = 3,
};
static inline void
__always_inline
do_set_elision_enable (int32_t elision_enable)
{
/* Enable elision if it's available in hardware. It's not necessary to check
if __libc_enable_secure isn't enabled since elision_enable will be set
according to the default, which is disabled. */
if (elision_enable == 1)
__pthread_force_elision = (GLRO (dl_hwcap2)
& PPC_FEATURE2_HAS_HTM) ? 1 : 0;
}
/* The pthread->elision_enable tunable is 0 or 1 indicating that elision
should be disabled or enabled respectively. The feature will only be used
if it's supported by the hardware. */
void
TUNABLE_CALLBACK (set_elision_enable) (tunable_val_t *valp)
{
int32_t elision_enable = (int32_t) valp->numval;
do_set_elision_enable (elision_enable);
}
#define TUNABLE_CALLBACK_FNDECL(__name, __type) \
static inline void \
__always_inline \
do_set_elision_ ## __name (__type value) \
{ \
__elision_aconf.__name = value; \
} \
void \
TUNABLE_CALLBACK (set_elision_ ## __name) (tunable_val_t *valp) \
{ \
__type value = (__type) (valp)->numval; \
do_set_elision_ ## __name (value); \
}
TUNABLE_CALLBACK_FNDECL (skip_lock_busy, int32_t);
TUNABLE_CALLBACK_FNDECL (skip_lock_internal_abort, int32_t);
TUNABLE_CALLBACK_FNDECL (skip_lock_out_of_tbegin_retries, int32_t);
TUNABLE_CALLBACK_FNDECL (try_tbegin, int32_t);
TUNABLE_CALLBACK_FNDECL (skip_trylock_internal_abort, int32_t);
/* Initialize elision. */
void
__lll_elision_init (void)
{
/* Elision depends on tunables and must be explicitly turned on by setting
the appropriate tunable on a supported platform. */
TUNABLE_GET (enable, int32_t,
TUNABLE_CALLBACK (set_elision_enable));
TUNABLE_GET (skip_lock_busy, int32_t,
TUNABLE_CALLBACK (set_elision_skip_lock_busy));
TUNABLE_GET (skip_lock_internal_abort, int32_t,
TUNABLE_CALLBACK (set_elision_skip_lock_internal_abort));
TUNABLE_GET (skip_lock_after_retries, int32_t,
TUNABLE_CALLBACK (set_elision_skip_lock_out_of_tbegin_retries));
TUNABLE_GET (tries, int32_t,
TUNABLE_CALLBACK (set_elision_try_tbegin));
TUNABLE_GET (skip_trylock_internal_abort, int32_t,
TUNABLE_CALLBACK (set_elision_skip_trylock_internal_abort));
/* Linux from 3.9 through 4.2 do not abort HTM transaction on syscalls,
instead it suspends the transaction and resumes it when returning to
usercode. The side-effects of the syscall will always remain visible,
even if the transaction is aborted. This is an issue when a transaction
is used along with futex syscall, on pthread_cond_wait for instance,
where futex might succeed but the transaction is rolled back leading
the condition variable object in an inconsistent state.
Glibc used to prevent it by always aborting a transaction before issuing
a syscall. Linux 4.2 also decided to abort active transaction in
syscalls which makes the glibc workaround superflours. Worse, glibc
transaction abortions leads to a performance issues on recent kernels.
So Lock Elision is just enabled when it has been explicitly set (either
by tunables of by a configure switch) and if kernel aborts HTM
transactions on syscalls (PPC_FEATURE2_HTM_NOSC) */
__pthread_force_elision = (__pthread_force_elision
&& GLRO (dl_hwcap2) & PPC_FEATURE2_HTM_NOSC);
if (!__pthread_force_elision)
__elision_aconf.try_tbegin = 0; /* Disable elision on rwlocks. */
}
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