Re: [PATCH v5 6/6] drm/syncobj: Add a fast path to drm_syncobj_array_find

Thu, 12 Jun 2025 04:07:06 -0700 


On 12/06/2025 08:21, Christian König wrote:

On 6/11/25 17:29, Tvrtko Ursulin wrote:


On 11/06/2025 15:21, Christian König wrote:

On 6/11/25 16:00, Tvrtko Ursulin wrote:

Running the Cyberpunk 2077 benchmark we can observe that the lookup helper
is relatively hot, but the 97% of the calls are for a single object. (~3%
for two points, and never more than three points. While a more trivial
workload like vkmark under Plasma is even more skewed to single point
lookups.)

Therefore lets add a fast path to bypass the kmalloc_array/kfree and use a
pre-allocated stack array for those cases.


Have you considered using memdup_user()? That's using a separate bucket IIRC 
and might give similar performance.


I haven't but I can try it. I would be surprised if it made a (positive) 
difference though.


Yeah, it's mostly for extra security I think.



On this topic, this discussion prompted me to quickly cook up some 
trivial cleanups for amdgpu to use memdup_user & co where it was easy. 
Series is on the mailing list but I did not copy you explicitly giving 
chance for someone else to notice it and off load you a bit.



And I realised I need to repeat the benchmarks anyway, since in v4 I had to 
stop doing access_ok+__get_user, after kernel test robot let me know 64-bit 
get_user is a not a thing on all platforms. I thought the gains are from 
avoiding allocations but, as you say, now I need to see if copy_from_user 
doesn't nullify them..


If that is still not sufficient I'm really wondering if we shouldn't have a 
macro for doing this. It's a really common use case as far as I can see.


Hmm macro for what exactly?


Like a macro which uses an array on the stack for small (<4) number of values 
and k(v)malloc() for large ones.

IIRC there is also a relatively new functionality which allows releasing the 
memory automatically when we leave the function.



Okay I will have a look at all those options. But it's going to the 
bottom of my priority pile so it might be a while.


Regards,

Tvrtko


Signed-off-by: Tvrtko Ursulin <tvrtko.ursu...@igalia.com>
Reviewed-by: Maíra Canal <mca...@igalia.com>
---
v2:
   * Added comments describing how the fast path arrays were sized.
   * Make container freeing criteria clearer by using a boolean.
---
   drivers/gpu/drm/drm_syncobj.c | 56 +++++++++++++++++++++++++++--------
   1 file changed, 44 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index be5905dca87f..65c301852f0d 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -1259,6 +1259,8 @@ EXPORT_SYMBOL(drm_timeout_abs_to_jiffies);
   static int drm_syncobj_array_find(struct drm_file *file_private,
                     u32 __user *handles,
                     uint32_t count,
+                  struct drm_syncobj **stack_syncobjs,
+                  u32 stack_count,
                     struct drm_syncobj ***syncobjs_out)
   {
       struct drm_syncobj **syncobjs;
@@ -1268,9 +1270,13 @@ static int drm_syncobj_array_find(struct drm_file 
*file_private,
       if (!access_ok(handles, count * sizeof(*handles)))
           return -EFAULT;
   -    syncobjs = kmalloc_array(count, sizeof(*syncobjs), GFP_KERNEL);
-    if (!syncobjs)
-        return -ENOMEM;
+    if (count > stack_count) {
+        syncobjs = kmalloc_array(count, sizeof(*syncobjs), GFP_KERNEL);
+        if (!syncobjs)
+            return -ENOMEM;
+    } else {
+        syncobjs = stack_syncobjs;
+    }
         for (i = 0; i < count; i++) {
           u32 handle;
@@ -1292,25 +1298,31 @@ static int drm_syncobj_array_find(struct drm_file 
*file_private,
   err_put_syncobjs:
       while (i-- > 0)
           drm_syncobj_put(syncobjs[i]);
-    kfree(syncobjs);
+
+    if (syncobjs != stack_syncobjs)
+        kfree(syncobjs);
         return ret;
   }
     static void drm_syncobj_array_free(struct drm_syncobj **syncobjs,
-                   uint32_t count)
+                   uint32_t count,
+                   bool free_container)
   {
       uint32_t i;
         for (i = 0; i < count; i++)
           drm_syncobj_put(syncobjs[i]);
-    kfree(syncobjs);
+
+    if (free_container)
+        kfree(syncobjs);
   }
     int
   drm_syncobj_wait_ioctl(struct drm_device *dev, void *data,
                  struct drm_file *file_private)
   {
+    struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
       struct drm_syncobj_wait *args = data;
       ktime_t deadline, *pdeadline = NULL;
       u32 count = args->count_handles;
@@ -1336,6 +1348,8 @@ drm_syncobj_wait_ioctl(struct drm_device *dev, void *data,
       ret = drm_syncobj_array_find(file_private,
                        u64_to_user_ptr(args->handles),
                        count,
+                     stack_syncobjs,
+                     ARRAY_SIZE(stack_syncobjs),
                        &syncobjs);
       if (ret < 0)
           return ret;
@@ -1354,7 +1368,7 @@ drm_syncobj_wait_ioctl(struct drm_device *dev, void *data,
                            &first,
                            pdeadline);
   -    drm_syncobj_array_free(syncobjs, count);
+    drm_syncobj_array_free(syncobjs, count, syncobjs != stack_syncobjs);
         if (timeout < 0)
           return timeout;
@@ -1368,6 +1382,7 @@ int
   drm_syncobj_timeline_wait_ioctl(struct drm_device *dev, void *data,
                   struct drm_file *file_private)
   {
+    struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
       struct drm_syncobj_timeline_wait *args = data;
       ktime_t deadline, *pdeadline = NULL;
       u32 count = args->count_handles;
@@ -1394,6 +1409,8 @@ drm_syncobj_timeline_wait_ioctl(struct drm_device *dev, 
void *data,
       ret = drm_syncobj_array_find(file_private,
                        u64_to_user_ptr(args->handles),
                        count,
+                     stack_syncobjs,
+                     ARRAY_SIZE(stack_syncobjs),
                        &syncobjs);
       if (ret < 0)
           return ret;
@@ -1412,7 +1429,7 @@ drm_syncobj_timeline_wait_ioctl(struct drm_device *dev, 
void *data,
                            &first,
                            pdeadline);
   -    drm_syncobj_array_free(syncobjs, count);
+    drm_syncobj_array_free(syncobjs, count, syncobjs != stack_syncobjs);
         if (timeout < 0)
           return timeout;
@@ -1529,6 +1546,7 @@ int
   drm_syncobj_reset_ioctl(struct drm_device *dev, void *data,
               struct drm_file *file_private)
   {
+    struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
       struct drm_syncobj_array *args = data;
       struct drm_syncobj **syncobjs;
       uint32_t i;
@@ -1546,6 +1564,8 @@ drm_syncobj_reset_ioctl(struct drm_device *dev, void 
*data,
       ret = drm_syncobj_array_find(file_private,
                        u64_to_user_ptr(args->handles),
                        args->count_handles,
+                     stack_syncobjs,
+                     ARRAY_SIZE(stack_syncobjs),
                        &syncobjs);
       if (ret < 0)
           return ret;
@@ -1553,7 +1573,8 @@ drm_syncobj_reset_ioctl(struct drm_device *dev, void 
*data,
       for (i = 0; i < args->count_handles; i++)
           drm_syncobj_replace_fence(syncobjs[i], NULL);
   -    drm_syncobj_array_free(syncobjs, args->count_handles);
+    drm_syncobj_array_free(syncobjs, args->count_handles,
+                   syncobjs != stack_syncobjs);
         return 0;
   }
@@ -1562,6 +1583,7 @@ int
   drm_syncobj_signal_ioctl(struct drm_device *dev, void *data,
                struct drm_file *file_private)
   {
+    struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
       struct drm_syncobj_array *args = data;
       struct drm_syncobj **syncobjs;
       uint32_t i;
@@ -1579,6 +1601,8 @@ drm_syncobj_signal_ioctl(struct drm_device *dev, void 
*data,
       ret = drm_syncobj_array_find(file_private,
                        u64_to_user_ptr(args->handles),
                        args->count_handles,
+                     stack_syncobjs,
+                     ARRAY_SIZE(stack_syncobjs),
                        &syncobjs);
       if (ret < 0)
           return ret;
@@ -1589,7 +1613,8 @@ drm_syncobj_signal_ioctl(struct drm_device *dev, void 
*data,
               break;
       }
   -    drm_syncobj_array_free(syncobjs, args->count_handles);
+    drm_syncobj_array_free(syncobjs, args->count_handles,
+                   syncobjs != stack_syncobjs);
         return ret;
   }
@@ -1598,6 +1623,7 @@ int
   drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data,
                     struct drm_file *file_private)
   {
+    struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
       struct drm_syncobj_timeline_array *args = data;
       uint64_t __user *points = u64_to_user_ptr(args->points);
       uint32_t i, j, count = args->count_handles;
@@ -1617,6 +1643,8 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, 
void *data,
       ret = drm_syncobj_array_find(file_private,
                        u64_to_user_ptr(args->handles),
                        count,
+                     stack_syncobjs,
+                     ARRAY_SIZE(stack_syncobjs),
                        &syncobjs);
       if (ret < 0)
           return ret;
@@ -1653,7 +1681,7 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, 
void *data,
   err_chains:
       kfree(chains);
   out:
-    drm_syncobj_array_free(syncobjs, count);
+    drm_syncobj_array_free(syncobjs, count, syncobjs != stack_syncobjs);
         return ret;
   }
@@ -1661,6 +1689,7 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, 
void *data,
   int drm_syncobj_query_ioctl(struct drm_device *dev, void *data,
                   struct drm_file *file_private)
   {
+    struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
       struct drm_syncobj_timeline_array *args = data;
       struct drm_syncobj **syncobjs;
       uint64_t __user *points = u64_to_user_ptr(args->points);
@@ -1679,6 +1708,8 @@ int drm_syncobj_query_ioctl(struct drm_device *dev, void 
*data,
       ret = drm_syncobj_array_find(file_private,
                        u64_to_user_ptr(args->handles),
                        args->count_handles,
+                     stack_syncobjs,
+                     ARRAY_SIZE(stack_syncobjs),
                        &syncobjs);
       if (ret < 0)
           return ret;
@@ -1722,7 +1753,8 @@ int drm_syncobj_query_ioctl(struct drm_device *dev, void 
*data,
           if (ret)
               break;
       }
-    drm_syncobj_array_free(syncobjs, args->count_handles);
+    drm_syncobj_array_free(syncobjs, args->count_handles,
+                   syncobjs != stack_syncobjs);
         return ret;
   }






























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