diff options
| author | Adhemerval Zanella <adhemerval.zanella@linaro.org> | 2018-08-27 09:42:50 -0300 |
|---|---|---|
| committer | Adhemerval Zanella <adhemerval.zanella@linaro.org> | 2018-09-21 10:18:03 -0700 |
| commit | f0458cf4f9ff3d870c43b624e6dccaaf657d5e83 (patch) | |
| tree | 92e48875fbeb81db1890ea1b8f75b5c7ec900ae1 /NEWS | |
| parent | 434d45fd70ac1a137d01b715ea99c03ce3c21b14 (diff) | |
| download | glibc-f0458cf4f9ff3d870c43b624e6dccaaf657d5e83.tar.gz | |
powerpc: Only enable TLE with PPC_FEATURE2_HTM_NOSC
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>
Diffstat (limited to 'NEWS')
| -rw-r--r-- | NEWS | 9 |
1 files changed, 9 insertions, 0 deletions
@@ -21,6 +21,15 @@ Major new features: * The reallocarray function is now declared under _DEFAULT_SOURCE, not just for _GNU_SOURCE, to match BSD environments. +* For powercp64le ABI, Transactional Lock Elision is now enabled iff kernel + indicates that it will abort the transaction prior to entering the kernel + (PPC_FEATURE2_HTM_NOSC on hwcap2). On older kernels the transaction is + suspended, and this caused some undefined side-effects issues by aborting + transactions manually. Glibc avoided it by abort transactions manually on + each syscall, but it lead to performance issues on newer kernels where the + HTM state is saved and restore lazily (the state being saved even when the + process actually does not use HTM). + Deprecated and removed features, and other changes affecting compatibility: * The glibc.tune tunable namespace has been renamed to glibc.cpu and the |