[PATCH] Add GLIBC_PTHREAD_ELISION_ENABLE tunable

["Paul E. Murphy" <murphyp at linux dot vnet dot ibm dot com>]

Some related discussions below:

https://sourceware.org/ml/libc-alpha/2015-09/msg00128.html
https://sourceware.org/ml/libc-alpha/2015-09/msg00065.html
https://sourceware.org/ml/libc-alpha/2015-09/msg00130.html

---8<---
This tunable controls the usage of transactional lock elision
within the nptl.  This also defines a new manual section to
describe the tunable, and any future tunables.

powerpc now disables TLE by default, and uses the tunable
to optionally enable the feature if desired.

2015-09-09  Paul E. Murphy  <murphyp@linux.vnet.ibm.com>

	* manual/Makefile (chapters): Add tunables.texi.
	* manual/into.texi: Add reference to Tunables.
	* manual/probes.texi: Enforce tunables.texi chapter location.
	* manual/tunables.texi: New file.
	* sysdeps/unix/sysv/linux/powerpc/elision-conf.c (elision-init):
	Add tunable to enable elision.
	* sysdeps/unix/sysv/linux/390/elision-conf.c (elision-init):
	Add tunable to disable Likewise.
	* sysdeps/unix/sysv/linux/x86/elision-conf.c (elision-init):
	Likewise.
---
 manual/Makefile                                |    3 +-
 manual/intro.texi                              |    3 ++
 manual/probes.texi                             |    2 +-
 manual/tunables.texi                           |   36 ++++++++++++++++++++++++
 sysdeps/unix/sysv/linux/powerpc/elision-conf.c |    6 +++-
 sysdeps/unix/sysv/linux/s390/elision-conf.c    |    5 +++
 sysdeps/unix/sysv/linux/x86/elision-conf.c     |    5 +++
 7 files changed, 57 insertions(+), 3 deletions(-)
 create mode 100644 manual/tunables.texi

diff --git a/manual/Makefile b/manual/Makefile
index cdb6763..6b2adb0 100644
--- a/manual/Makefile
+++ b/manual/Makefile
@@ -38,7 +38,8 @@ chapters = $(addsuffix .texi, \
 		       message search pattern io stdio llio filesys	\
 		       pipe socket terminal syslog math arith time	\
 		       resource setjmp signal startup process ipc job	\
-		       nss users sysinfo conf crypt debug threads probes)
+		       nss users sysinfo conf crypt debug threads probes\
+		       tunables)
 add-chapters = $(wildcard $(foreach d, $(add-ons), ../$d/$d.texi))
 appendices = lang.texi header.texi install.texi maint.texi platform.texi \
 	     contrib.texi
diff --git a/manual/intro.texi b/manual/intro.texi
index d4045f2..bcd4f5b 100644
--- a/manual/intro.texi
+++ b/manual/intro.texi
@@ -1475,6 +1475,9 @@ various operating system limits.  Most of these parameters are provided for
 compatibility with POSIX.
 
 @item
+@ref{Tunables}, describes the tunables and their usage.
+
+@item
 @ref{Language Features}, contains information about library support for
 standard parts of the C language, including things like the @code{sizeof}
 operator and the symbolic constant @code{NULL}, how to write functions
diff --git a/manual/probes.texi b/manual/probes.texi
index 7dd56d8..110dc8d 100644
--- a/manual/probes.texi
+++ b/manual/probes.texi
@@ -1,5 +1,5 @@
 @node Internal Probes
-@c @node Internal Probes, , POSIX Threads, Top
+@c @node Internal Probes, Tunables, POSIX Threads, Top
 @c %MENU% Probes to monitor libc internal behavior
 @chapter Internal probes
 
diff --git a/manual/tunables.texi b/manual/tunables.texi
new file mode 100644
index 0000000..140fc99
--- /dev/null
+++ b/manual/tunables.texi
@@ -0,0 +1,36 @@
+@node Tunables
+@c @node Tunables, , , Top
+@c %MENU% Available tuning parameters
+@chapter Tunables
+
+This chapter describes the @glibcadj{} tunables available to the user.
+
+A tunable is an environment variable prefixed with GLIBC_ which may alter
+the performance characteristics of a feature provided by the @glibcadj{}.  It
+should be noted that tunables behave as any other environment variable.  Thus
+the user is cautioned about the effects these may have when inherited.
+
+@strong{Warning}, tunables are not part of the @glibcadj{} stable ABI, and they
+are subject to change or removal across releases.
+
+@menu
+* POSIX Thread Tunables:: Tunables controlling the default behavior of pthreads
+@end menu
+
+@node POSIX Thread Tunables
+@section POSIX Thread Tunables
+
+@subsection @code{GLIBC_PTHREAD_ELISION_ENABLE}
+This variable controls whether transactional lock elision is enabled at
+runtime.  This feature may not be available on all platforms, and requires a
+library explicitly built with support.  The default state of this tunable is
+dependent on the architecture and the configuration of the library.  For
+instance, powerpc will not elide locks without explicitly setting this
+tunable.  The following values are honored by this variable:
+
+@table @code
+@item yes
+Enable lock elision if supported by the hardware and underlying platform
+@item no
+Disable all usage of lock elision
+@end table
diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-conf.c b/sysdeps/unix/sysv/linux/powerpc/elision-conf.c
index 5341222..13d5b6d 100644
--- a/sysdeps/unix/sysv/linux/powerpc/elision-conf.c
+++ b/sysdeps/unix/sysv/linux/powerpc/elision-conf.c
@@ -61,7 +61,11 @@ elision_init (int argc __attribute__ ((unused)),
 {
 #ifdef ENABLE_LOCK_ELISION
   int elision_available = (GLRO (dl_hwcap2) & PPC_FEATURE2_HAS_HTM) ? 1 : 0;
-  __pthread_force_elision = __libc_enable_secure ? 0 : elision_available;
+
+  /* Check environment to see if we should enable LE.  */
+  for (; *environ != NULL; environ++)
+    if (strcmp (*environ, "GLIBC_PTHREAD_ELISION_ENABLE=yes") == 0)
+      __pthread_force_elision = __libc_enable_secure ? 0 : elision_available;
 #endif
   if (!__pthread_force_elision)
     /* Disable elision on rwlocks.  */
diff --git a/sysdeps/unix/sysv/linux/s390/elision-conf.c b/sysdeps/unix/sysv/linux/s390/elision-conf.c
index e1ff599..f50df9f 100644
--- a/sysdeps/unix/sysv/linux/s390/elision-conf.c
+++ b/sysdeps/unix/sysv/linux/s390/elision-conf.c
@@ -64,6 +64,11 @@ elision_init (int argc __attribute__ ((unused)),
      When false elision is never attempted.  */
   int elision_available = (GLRO (dl_hwcap) & HWCAP_S390_TE) ? 1 : 0;
 
+  /* Check environment to see if we should disable LE.  */
+  for (; *environ != NULL; environ++)
+    if (strcmp (*environ, "GLIBC_PTHREAD_ELISION_ENABLE=no") == 0)
+      __pthread_force_elision = 0;
+
   __pthread_force_elision = __libc_enable_secure ? 0 : elision_available;
 }
 
diff --git a/sysdeps/unix/sysv/linux/x86/elision-conf.c b/sysdeps/unix/sysv/linux/x86/elision-conf.c
index 4a73382..73cd37e 100644
--- a/sysdeps/unix/sysv/linux/x86/elision-conf.c
+++ b/sysdeps/unix/sysv/linux/x86/elision-conf.c
@@ -65,6 +65,11 @@ elision_init (int argc __attribute__ ((unused)),
   __elision_available = HAS_CPU_FEATURE (RTM);
 #ifdef ENABLE_LOCK_ELISION
   __pthread_force_elision = __libc_enable_secure ? 0 : __elision_available;
+
+  /* Check environment to see if we should disable LE.  */
+  for (; *environ != NULL; environ++)
+    if (strcmp (*environ, "GLIBC_PTHREAD_ELISION_ENABLE=no") == 0)
+      __pthread_force_elision = 0;
 #endif
   if (!HAS_CPU_FEATURE (RTM))
     __elision_aconf.retry_try_xbegin = 0; /* Disable elision on rwlocks */
-- 
1.7.1