[Mesa-dev] [RFC 1/3] mesa: Add new fast mtx_t mutex type for basic use cases

[Timothy Arceri]

While modern pthread mutexes are very fast, they still incur a call to an
external DSO and overhead of the generality and features of pthread mutexes.
Most mutexes in mesa only needs lock/unlock, and the idea here is that we can
inline the atomic operation and make the fast case just two intructions.
Mutexes are subtle and finicky to implement, so we carefully copy the
implementation from Ulrich Dreppers well-written and well-reviewed paper:

  "Futexes Are Tricky"
  http://www.akkadia.org/drepper/futex.pdf

We implement "mutex3", which gives us a mutex that has no syscalls on
uncontended lock or unlock.  Further, the uncontended case boils down to a
cmpxchg and an untaken branch and the uncontended unlock is just a locked decr
and an untaken branch.  We use __builtin_expect() to indicate that contention
is unlikely so that gcc will put the contention code out of the main code
flow.

A fast mutex only supports lock/unlock, can't be recursive or used with
condition variables.  We keep the pthread mutex implementation around as
full_mtx_t for the few places where we use condition variables or recursive
locking.  For platforms or compilers where futex and atomics aren't available,
mtx_t falls back to the pthread mutex.

The pthread mutex lock/unlock overhead shows up on benchmarks for CPU bound
applications.  Most CPU bound cases are helped and some of our internal
bind_buffer_object heavy benchmarks gain up to 10%.

Signed-off-by: Kristian Høgsberg <krh at bitplanet.net>
Signed-off-by: Timothy Arceri <tarceri at itsqueeze.com>
---
 configure.ac          |   3 ++
 src/util/simple_mtx.h | 146 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 149 insertions(+)
 create mode 100644 src/util/simple_mtx.h

diff --git a/configure.ac b/configure.ac
index 903a3979d4..47fe427d19 100644
--- a/configure.ac
+++ b/configure.ac
@@ -887,20 +887,23 @@ linux* | cygwin* | darwin* | solaris* | *-gnu* | gnu*)
     ;;
 * )
     pthread_stubs_possible="yes"
     ;;
 esac
 
 if test "x$pthread_stubs_possible" = xyes; then
     PKG_CHECK_MODULES(PTHREADSTUBS, pthread-stubs >= 0.4)
 fi
 
+dnl Check for futex for fast inline simple_mtx_t.
+AC_CHECK_HEADER([linux/futex.h], [DEFINES="$DEFINES -DHAVE_FUTEX"])
+
 dnl SELinux awareness.
 AC_ARG_ENABLE([selinux],
     [AS_HELP_STRING([--enable-selinux],
         [Build SELinux-aware Mesa @<:@default=disabled@:>@])],
     [MESA_SELINUX="$enableval"],
     [MESA_SELINUX=no])
 if test "x$enable_selinux" = "xyes"; then
     PKG_CHECK_MODULES([SELINUX], [libselinux], [],
         [AC_CHECK_HEADER([selinux/selinux.h],[],
                          [AC_MSG_ERROR([SELinux headers not found])])
diff --git a/src/util/simple_mtx.h b/src/util/simple_mtx.h
new file mode 100644
index 0000000000..bd365ac237
--- /dev/null
+++ b/src/util/simple_mtx.h
@@ -0,0 +1,146 @@
+/*
+ * Copyright © 2015 Intel
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#if defined(__GNUC__) && defined(HAVE_FUTEX)
+
+/* mtx_t - Fast, simple mutex
+ *
+ * While modern pthread mutexes are very fast (implemented using futex), they
+ * still incur a call to an external DSO and overhead of the generality and
+ * features of pthread mutexes.  Most mutexes in mesa only needs lock/unlock,
+ * and the idea here is that we can inline the atomic operation and make the
+ * fast case just two intructions.  Mutexes are subtle and finicky to
+ * implement, so we carefully copy the implementation from Ulrich Dreppers
+ * well-written and well-reviewed paper:
+ *
+ *   "Futexes Are Tricky"
+ *   http://www.akkadia.org/drepper/futex.pdf
+ *
+ * We implement "mutex3", which gives us a mutex that has no syscalls on
+ * uncontended lock or unlock.  Further, the uncontended case boils down to a
+ * locked cmpxchg and an untaken branch, the uncontended unlock is just a
+ * locked decr and an untaken branch.  We use __builtin_expect() to indicate
+ * that contention is unlikely so that gcc will put the contention code out of
+ * the main code flow.
+ *
+ * A fast mutex only supports lock/unlock, can't be recursive or used with
+ * condition variables.
+ */
+
+typedef struct {
+   uint32_t val;
+} simple_mtx_t;
+
+#define _SIMPLE_MTX_INITIALIZER_NP { 0 }
+
+#include <stdint.h>
+#include <values.h>
+#include <linux/futex.h>
+#include <sys/time.h>
+#include <sys/syscall.h>
+
+static inline long sys_futex(void *addr1, int op, int val1, struct timespec *timeout, void *addr2, int val3)
+{
+   return syscall(SYS_futex, addr1, op, val1, timeout, addr2, val3);
+}
+
+static inline int futex_wake(uint32_t *addr) {
+   return sys_futex(addr, FUTEX_WAKE, 1, NULL, NULL, 0);
+}
+
+static inline int futex_wait(uint32_t *addr, int32_t value) {
+   return sys_futex(addr, FUTEX_WAIT, value, NULL, NULL, 0);
+}
+
+static inline void
+simple_mtx_init(simple_mtx_t *mtx, int type)
+{
+   assert(type == mtx_plain);
+
+   mtx->val = 0;
+}
+
+static inline void
+simple_mtx_destroy(simple_mtx_t *mtx)
+{
+}
+
+static inline void
+simple_mtx_lock(simple_mtx_t *mtx)
+{
+   uint32_t c;
+
+   c = __sync_val_compare_and_swap(&mtx->val, 0, 1);
+   if (__builtin_expect(c != 0, 0)) {
+      if (c != 2)
+         c = __sync_lock_test_and_set(&mtx->val, 2);
+      while (c != 0) {
+         futex_wait(&mtx->val, 2);
+         c = __sync_lock_test_and_set(&mtx->val, 2);
+      }
+   }
+}
+
+static inline void
+simple_mtx_unlock(simple_mtx_t *mtx)
+{
+   uint32_t c;
+
+   c = __sync_fetch_and_sub(&mtx->val, 1);
+   if (__builtin_expect(c != 1, 0)) {
+      mtx->val = 0;
+      futex_wake(&mtx->val);
+   }
+}
+
+#else
+
+typedef simple_mtx_t mtx_t;
+
+#define _SIMPLE_MTX_INITIALIZER_NP _MTX_INITIALIZER_NP
+
+static inline void
+simple_mtx_init(simple_mtx_t *mtx, int type)
+{
+   mtx_init(mtx, type);
+}
+
+static inline void
+simple_mtx_destroy(simple_mtx_t *mtx)
+{
+   mtx_destroy(mtx);
+}
+
+static inline void
+simple_mtx_lock(simple_mtx_t *mtx)
+{
+   mtx_lock(mtx);
+}
+
+static inline void
+simple_mtx_unlock(simple_mtx_t *mtx)
+{
+   mtx_unlock(mtx);
+}
+ 
+#endif
-- 
2.13.6