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powerpc: Only enable TLE with PPC_FEATURE2_HTM_NOSC

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Linux from 3.9 through 4.2 does not abort HTM transaction on syscalls,
instead it suspend and resume it when leaving the kernel.  The
side-effects of the syscall will always remain visible, even if the
transaction is aborted.  This is an issue when transaction is used along
with futex syscall, on pthread_cond_wait for instance, where the futex
call might succeed but the transaction is rolled back leading the
pthread_cond 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 superfluous.  Worse, glibc
transaction abortion leads to a performance issue on recent kernels
where the HTM state is saved/restore lazily (v4.9).  By aborting a
transaction on every syscalls, regardless whether a transaction has being
initiated before, GLIBS makes the kernel always save/restore HTM state
(it can not even lazily disable it after a certain number of syscall
iterations).

Because of this shortcoming, Transactional 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).  It is reported that using simple benchmark [1],
the context-switch is about 5% faster by not issuing a tabort in every
syscall in newer kernels.

Checked on powerpc64le-linux-gnu with 4.4.0 kernel (Ubuntu 16.04).

	* NEWS: Add note about new TLE support on powerpc64le.
	* sysdeps/powerpc/nptl/tcb-offsets.sym (TM_CAPABLE): Remove.
	* sysdeps/powerpc/nptl/tls.h (tcbhead_t): Rename tm_capable to
	__ununsed1.
	(TLS_INIT_TP, TLS_DEFINE_INIT_TP): Remove tm_capable setup.
	(THREAD_GET_TM_CAPABLE, THREAD_SET_TM_CAPABLE): Remove macros.
	* sysdeps/powerpc/powerpc32/sysdep.h,
	sysdeps/powerpc/powerpc64/sysdep.h (ABORT_TRANSACTION_IMPL,
	ABORT_TRANSACTION): Remove macros.
	* sysdeps/powerpc/sysdep.h (ABORT_TRANSACTION): Likewise.
	* sysdeps/unix/sysv/linux/powerpc/elision-conf.c (elision_init): Set
	__pthread_force_elision iff PPC_FEATURE2_HTM_NOSC is set.
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/sysdep.h,
	sysdeps/unix/sysv/linux/powerpc/powerpc64/sysdep.h
	sysdeps/unix/sysv/linux/powerpc/syscall.S (ABORT_TRANSACTION): Remove
	usage.
	* sysdeps/unix/sysv/linux/powerpc/not-errno.h: Remove file.

Reported-by: Breno Leitão <leitao@debian.org>
This commit is contained in:
Adhemerval Zanella
2018-08-27 09:42:50 -03:00
parent 434d45fd70
commit f0458cf4f9
12 changed files with 50 additions and 100 deletions
Download Patch File Download Diff File Expand all files Collapse all files

20
sysdeps/unix/sysv/linux/powerpc/elision-conf.c
View File

@ -127,6 +127,26 @@ elision_init (int argc __attribute__ ((unused)),
TUNABLE_CALLBACK (set_elision_skip_trylock_internal_abort));
#endif
/* 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 explict 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. */
}

30
sysdeps/unix/sysv/linux/powerpc/not-errno.h
View File

@ -1,30 +0,0 @@
/* Syscall wrapper that do not set errno. Linux powerpc version.
Copyright (C) 2018 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
<http://www.gnu.org/licenses/>. */
/* __access_noerrno is used during process initialization in elf/dl-tunables.c
before the TCB is initialized, prohibiting the usage of
ABORT_TRANSACTION. */
#undef ABORT_TRANSACTION
#define ABORT_TRANSACTION
#include "sysdeps/unix/sysv/linux/not-errno.h"
/* Recover ABORT_TRANSACTION's previous value, in order to not affect
other syscalls. */
#undef ABORT_TRANSACTION
#define ABORT_TRANSACTION ABORT_TRANSACTION_IMPL

1
sysdeps/unix/sysv/linux/powerpc/powerpc32/sysdep.h
View File

@ -109,7 +109,6 @@
register long int r11 __asm__ ("r11"); \
register long int r12 __asm__ ("r12"); \
LOADARGS_##nr(name, args); \
ABORT_TRANSACTION; \
__asm__ __volatile__ \
("sc \n\t" \
"mfcr %0" \

1
sysdeps/unix/sysv/linux/powerpc/powerpc64/sysdep.h
View File

@ -131,7 +131,6 @@
register long int r7 __asm__ ("r7"); \
register long int r8 __asm__ ("r8"); \
LOADARGS_##nr (name, ##args); \
ABORT_TRANSACTION; \
__asm__ __volatile__ \
("sc\n\t" \
"mfcr %0\n\t" \

1
sysdeps/unix/sysv/linux/powerpc/syscall.S
View File

@ -18,7 +18,6 @@
#include <sysdep.h>
ENTRY (syscall)
ABORT_TRANSACTION
mr r0,r3
mr r3,r4
mr r4,r5