powerpc: Only enable TLE with PPC_FEATURE2_HTM_NOSC

System Internals / glibc - Adhemerval Zanella [linaro.org] - 21 September 2018 17:18 EDT

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.

f0458cf4f9 powerpc: Only enable TLE with PPC_FEATURE2_HTM_NOSC
ChangeLog | 20 +++++++++++++++
NEWS | 9 +++++++
sysdeps/powerpc/nptl/tcb-offsets.sym | 1 -
sysdeps/powerpc/nptl/tls.h | 13 +---------
sysdeps/powerpc/powerpc32/sysdep.h | 17 ------------
sysdeps/powerpc/powerpc64/sysdep.h | 17 ------------
sysdeps/powerpc/sysdep.h | 20 ---------------
sysdeps/unix/sysv/linux/powerpc/elision-conf.c | 20 +++++++++++++++
sysdeps/unix/sysv/linux/powerpc/not-errno.h | 30 ----------------------
sysdeps/unix/sysv/linux/powerpc/powerpc32/sysdep.h | 1 -
sysdeps/unix/sysv/linux/powerpc/powerpc64/sysdep.h | 1 -
sysdeps/unix/sysv/linux/powerpc/syscall.S | 1 -
12 files changed, 50 insertions(+), 100 deletions(-)

Upstream: sourceware.org


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