[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.ursulin at 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 <mcanal at igalia.com>

Best Regards,
- Maíra

> +		kfree(entries);
>   
>   	return timeout;
>   }