From: Paul Murphy <murphyp@linux.vnet.ibm.com>
Date: Thu, 3 Sep 2015 18:40:21 +0000 (-0500)
Subject: powerpc: Optimize lock elision for pthread_mutex_t
X-Git-Tag: glibc-2.23~321
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=fadd2ad9cc36115440d50b0eae9299e65988917d;p=thirdparty%2Fglibc.git

powerpc: Optimize lock elision for pthread_mutex_t

With TLE enabled, the adapt count variable update incurs
an 8% overhead before entering the critical section of an
elided mutex.

Instead, if it is done right after leaving the critical
section, this serialization can be avoided.

This alters the existing behavior of __lll_trylock_elision
as it will only decrement the adapt_count if it successfully
acquires the lock.

	* sysdeps/unix/sysv/linux/powerpc/elision-lock.c
	(__lll_lock_elision): Remove adapt_count decrement...
	* sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
	(__lll_trylock_elision): Likewise.
	* sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
	(__lll_unlock_elision): ... to here. And utilize
	new adapt_count parameter.
	* sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
	(__lll_unlock_elision): Update to include adapt_count
	parameter.
	(lll_unlock_elision): Pass pointer to adapt_count
	variable.
---

diff --git a/ChangeLog b/ChangeLog
index 6a743e454b7..917c4649d93 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,18 @@
+2015-10-15  Paul E. Murphy  <murphyp@linux.vnet.ibm.com>
+
+	* sysdeps/unix/sysv/linux/powerpc/elision-lock.c
+	(__lll_lock_elision): Remove adapt_count decrement...
+	* sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
+	(__lll_trylock_elision): Likewise.
+	* sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
+	(__lll_unlock_elision): ... to here. And utilize
+	new adapt_count parameter.
+	* sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
+	(__lll_unlock_elision): Update to include adapt_count
+	parameter.
+	(lll_unlock_elision): Pass pointer to adapt_count
+	variable.
+
 2015-10-15  Paul E. Murphy  <murphyp@linux.vnet.ibm.com>
 
 	* nptl/pthread_mutex_unlock.c (lll_unlock_elision):
diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-lock.c b/sysdeps/unix/sysv/linux/powerpc/elision-lock.c
index 26d272e619f..37627323798 100644
--- a/sysdeps/unix/sysv/linux/powerpc/elision-lock.c
+++ b/sysdeps/unix/sysv/linux/powerpc/elision-lock.c
@@ -47,7 +47,6 @@ __lll_lock_elision (int *lock, short *adapt_count, EXTRAARG int pshared)
 {
   if (*adapt_count > 0)
     {
-      (*adapt_count)--;
       goto use_lock;
     }
 
diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-trylock.c b/sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
index 7b6d1b9b983..440939cda15 100644
--- a/sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
+++ b/sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
@@ -36,7 +36,6 @@ __lll_trylock_elision (int *futex, short *adapt_count)
   /* Only try a transaction if it's worth it.  */
   if (*adapt_count > 0)
     {
-      (*adapt_count)--;
       goto use_lock;
     }
 
diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c b/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
index f04c3393831..72b893d9f41 100644
--- a/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
+++ b/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
@@ -21,12 +21,20 @@
 #include "htm.h"
 
 int
-__lll_unlock_elision(int *lock, int pshared)
+__lll_unlock_elision (int *lock, short *adapt_count, int pshared)
 {
   /* When the lock was free we're in a transaction.  */
   if (*lock == 0)
     __builtin_tend (0);
   else
-    lll_unlock ((*lock), pshared);
+    {
+      lll_unlock ((*lock), pshared);
+
+      /* Update the adapt count AFTER completing the critical section.
+         Doing this here prevents unneeded stalling when entering
+         a critical section.  Saving about 8% runtime on P8.  */
+      if (*adapt_count > 0)
+	(*adapt_count)--;
+    }
   return 0;
 }
diff --git a/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h b/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
index 16479e77530..6769c253cee 100644
--- a/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
+++ b/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
@@ -32,7 +32,7 @@ extern int __lll_timedlock_elision
 extern int __lll_lock_elision (int *futex, short *adapt_count, int private)
   attribute_hidden;
 
-extern int __lll_unlock_elision(int *lock, int private)
+extern int __lll_unlock_elision (int *lock, short *adapt_count, int private)
   attribute_hidden;
 
 extern int __lll_trylock_elision(int *lock, short *adapt_count)
@@ -41,7 +41,7 @@ extern int __lll_trylock_elision(int *lock, short *adapt_count)
 #define lll_lock_elision(futex, adapt_count, private) \
   __lll_lock_elision (&(futex), &(adapt_count), private)
 #define lll_unlock_elision(futex, adapt_count, private) \
-  __lll_unlock_elision (&(futex), private)
+  __lll_unlock_elision (&(futex), &(adapt_count), private)
 #define lll_trylock_elision(futex, adapt_count) \
   __lll_trylock_elision (&(futex), &(adapt_count))