Re: [PATCH 6/7] drm/syncobj: Add a fast path to drm_syncobj_array_wait_timeout

[Maíra Canal]

Hi Tvrtko,

On 18/03/25 12:54, Tvrtko Ursulin wrote:

Running the Cyberpunk 2077 benchmark we can observe that waiting on DRM
sycobjs is relatively hot, but the 96% of the calls are for a single
object. (~4% 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 waits.)

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

Signed-off-by: Tvrtko Ursulin <tvrtko.ursu...@igalia.com>
---
  drivers/gpu/drm/drm_syncobj.c | 16 ++++++++++++----
  1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index b4563c696056..94932b89298f 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -1035,6 +1035,7 @@ static signed long drm_syncobj_array_wait_timeout(struct 
drm_syncobj **syncobjs,
                                                  uint32_t *idx,
                                                  ktime_t *deadline)
  {
+       struct syncobj_wait_entry stack_entries[4];

Can't you initialize as

struct syncobj_wait_entry stack_entries[4] = {0};

?

        struct syncobj_wait_entry *entries;
        uint32_t signaled_count, i;
        struct dma_fence *fence;
@@ -1049,9 +1050,14 @@ static signed long drm_syncobj_array_wait_timeout(struct 
drm_syncobj **syncobjs,
            !access_ok(user_points, count * sizeof(*user_points)))
                return -EFAULT;

- entries = kcalloc(count, sizeof(*entries), GFP_KERNEL);

-       if (!entries)
-               return -ENOMEM;
+       if (count > ARRAY_SIZE(stack_entries)) {
+               entries = kcalloc(count, sizeof(*entries), GFP_KERNEL);
+               if (!entries)
+                       return -ENOMEM;
+       } else {
+               memset(stack_entries, 0, sizeof(stack_entries));

Then, you can avoid this step.

+               entries = stack_entries;
+       }

/* Walk the list of sync objects and initialize entries. We do

         * this up-front so that we can properly return -EINVAL if there is
@@ -1174,7 +1180,9 @@ static signed long drm_syncobj_array_wait_timeout(struct 
drm_syncobj **syncobjs,
                                                  &entries[i].fence_cb);
                dma_fence_put(entries[i].fence);
        }
-       kfree(entries);
+
+       if (entries != stack_entries)

You can initialize `entries = NULL` and avoid this step.

Anyway,

Reviewed-by: Maíra Canal <mca...@igalia.com>

Best Regards,
- Maíra

+               kfree(entries);

return timeout;

  }