[Mesa-dev] [PATCH] mesa: Use atomics for buffer objects reference counts.

[Mathias.Froehlich at gmx.net]

From: Mathias Fröhlich <mathias.froehlich at web.de>

Hi all,

please review

best

Mathias


The mutex is currently used for reference counting and updating
the minmax index cache.
The change uses atomics directly for reference counting and
the mutex for the minmax cache.
This is safe since the reference count is not modified beside
in _mesa_reference_buffer_object where atomics aim to be used.
While using the minmax cache, the calling code holds a reference
to the buffer object. Thus unreferencing or even referencing the
buffer object does not need to be serialized with accessing
the minmax cache.
The change reduces the time _mesa_reference_buffer_object_ takes
by about a factor of two when looking at perf results for some
of my favorite use cases.

Signed-off-by: Mathias Fröhlich <Mathias.Froehlich at web.de>
---
 src/mesa/main/bufferobj.c       | 22 ++++++----------------
 src/mesa/main/mtypes.h          |  2 +-
 src/mesa/vbo/vbo_minmax_index.c |  8 ++++----
 3 files changed, 11 insertions(+), 21 deletions(-)

diff --git a/src/mesa/main/bufferobj.c b/src/mesa/main/bufferobj.c
index c1dfdfba82..67f9cd0a90 100644
--- a/src/mesa/main/bufferobj.c
+++ b/src/mesa/main/bufferobj.c
@@ -46,6 +46,7 @@
 #include "texstore.h"
 #include "transformfeedback.h"
 #include "varray.h"
+#include "util/u_atomic.h"
 
 
 /* Debug flags */
@@ -471,7 +472,7 @@ _mesa_delete_buffer_object(struct gl_context *ctx,
    bufObj->RefCount = -1000;
    bufObj->Name = ~0;
 
-   simple_mtx_destroy(&bufObj->Mutex);
+   simple_mtx_destroy(&bufObj->MinMaxCacheMutex);
    free(bufObj->Label);
    free(bufObj);
 }
@@ -490,16 +491,9 @@ _mesa_reference_buffer_object_(struct gl_context *ctx,
 {
    if (*ptr) {
       /* Unreference the old buffer */
-      GLboolean deleteFlag = GL_FALSE;
       struct gl_buffer_object *oldObj = *ptr;
 
-      simple_mtx_lock(&oldObj->Mutex);
-      assert(oldObj->RefCount > 0);
-      oldObj->RefCount--;
-      deleteFlag = (oldObj->RefCount == 0);
-      simple_mtx_unlock(&oldObj->Mutex);
-
-      if (deleteFlag) {
+      if (p_atomic_dec_zero(&oldObj->RefCount)) {
 	 assert(ctx->Driver.DeleteBuffer);
          ctx->Driver.DeleteBuffer(ctx, oldObj);
       }
@@ -510,12 +504,8 @@ _mesa_reference_buffer_object_(struct gl_context *ctx,
 
    if (bufObj) {
       /* reference new buffer */
-      simple_mtx_lock(&bufObj->Mutex);
-      assert(bufObj->RefCount > 0);
-
-      bufObj->RefCount++;
+      p_atomic_inc(&bufObj->RefCount);
       *ptr = bufObj;
-      simple_mtx_unlock(&bufObj->Mutex);
    }
 }
 
@@ -547,11 +537,11 @@ _mesa_initialize_buffer_object(struct gl_context *ctx,
                                GLuint name)
 {
    memset(obj, 0, sizeof(struct gl_buffer_object));
-   simple_mtx_init(&obj->Mutex, mtx_plain);
    obj->RefCount = 1;
    obj->Name = name;
    obj->Usage = GL_STATIC_DRAW_ARB;
 
+   simple_mtx_init(&obj->MinMaxCacheMutex, mtx_plain);
    if (get_no_minmax_cache())
       obj->UsageHistory |= USAGE_DISABLE_MINMAX_CACHE;
 }
@@ -870,7 +860,7 @@ _mesa_init_buffer_objects( struct gl_context *ctx )
    GLuint i;
 
    memset(&DummyBufferObject, 0, sizeof(DummyBufferObject));
-   simple_mtx_init(&DummyBufferObject.Mutex, mtx_plain);
+   simple_mtx_init(&DummyBufferObject.MinMaxCacheMutex, mtx_plain);
    DummyBufferObject.RefCount = 1000*1000*1000; /* never delete */
 
    _mesa_reference_buffer_object(ctx, &ctx->Array.ArrayBufferObj,
diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index 3a67d43420..b6d606386e 100644
--- a/src/mesa/main/mtypes.h
+++ b/src/mesa/main/mtypes.h
@@ -1443,7 +1443,6 @@ typedef enum
  */
 struct gl_buffer_object
 {
-   simple_mtx_t Mutex;
    GLint RefCount;
    GLuint Name;
    GLchar *Label;       /**< GL_KHR_debug */
@@ -1464,6 +1463,7 @@ struct gl_buffer_object
    struct gl_buffer_mapping Mappings[MAP_COUNT];
 
    /** Memoization of min/max index computations for static index buffers */
+   simple_mtx_t MinMaxCacheMutex;
    struct hash_table *MinMaxCache;
    unsigned MinMaxCacheHitIndices;
    unsigned MinMaxCacheMissIndices;
diff --git a/src/mesa/vbo/vbo_minmax_index.c b/src/mesa/vbo/vbo_minmax_index.c
index c9d2020167..d1298dcdc3 100644
--- a/src/mesa/vbo/vbo_minmax_index.c
+++ b/src/mesa/vbo/vbo_minmax_index.c
@@ -115,7 +115,7 @@ vbo_get_minmax_cached(struct gl_buffer_object *bufferObj,
    if (!vbo_use_minmax_cache(bufferObj))
       return GL_FALSE;
 
-   simple_mtx_lock(&bufferObj->Mutex);
+   simple_mtx_lock(&bufferObj->MinMaxCacheMutex);
 
    if (bufferObj->MinMaxCacheDirty) {
       /* Disable the cache permanently for this BO if the number of hits
@@ -166,7 +166,7 @@ out_invalidate:
    }
 
 out_disable:
-   simple_mtx_unlock(&bufferObj->Mutex);
+   simple_mtx_unlock(&bufferObj->MinMaxCacheMutex);
    return found;
 }
 
@@ -184,7 +184,7 @@ vbo_minmax_cache_store(struct gl_context *ctx,
    if (!vbo_use_minmax_cache(bufferObj))
       return;
 
-   simple_mtx_lock(&bufferObj->Mutex);
+   simple_mtx_lock(&bufferObj->MinMaxCacheMutex);
 
    if (!bufferObj->MinMaxCache) {
       bufferObj->MinMaxCache =
@@ -223,7 +223,7 @@ vbo_minmax_cache_store(struct gl_context *ctx,
       free(entry);
 
 out:
-   simple_mtx_unlock(&bufferObj->Mutex);
+   simple_mtx_unlock(&bufferObj->MinMaxCacheMutex);
 }
 
 
-- 
2.14.3