Re: [PATCH 1/2] drm/sched: add drm_sched_prealloc_dependency_slots

[Tvrtko Ursulin <tvrtko.ursulin@igalia.com>]

On 18/03/2025 12:03, Christian König wrote:
> The problem is that drivers sometimes need to add dependencies without
> allocating any memory.
> 
> Add a function which preallocates slots by inserting signaled stub fences
> into the dependency array.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>   drivers/gpu/drm/scheduler/sched_main.c | 41 ++++++++++++++++++++++++--
>   include/drm/gpu_scheduler.h            |  2 ++
>   2 files changed, 40 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 4d4219fbe49d..2085eea89513 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -852,6 +852,38 @@ void drm_sched_job_arm(struct drm_sched_job *job)
>   }
>   EXPORT_SYMBOL(drm_sched_job_arm);
>   
> +/**
> + * drm_sched_job_prealloc_dependency_slots - avoid ENOMEM on adding dependencies
> + * @job: scheduler job where dependencies will be added
> + * @num_deps: number of dependencies to preallocate slots for
> + *
> + * Preallocate memory so that no ENOMEM can come later while adding
> + * dependencies.
> + *
> + * Return:
> + * 0 on success, or an error on failing to expand the array.
> + */
> +int drm_sched_job_prealloc_dependency_slots(struct drm_sched_job *job,
> +					    unsigned int num_deps)
> +{
> +	struct dma_fence *fence;
> +	u32 id = 0;
> +	int ret;
> +
> +	while (num_deps--) {
> +		fence = dma_fence_get_stub();
> +		ret = xa_alloc(&job->dependencies, &id, fence, xa_limit_32b,
> +			       GFP_KERNEL);
> +		if (ret != 0) {
> +			dma_fence_put(fence);
> +			return ret;
> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_sched_job_prealloc_dependency_slots);
> +
>   /**
>    * drm_sched_job_add_dependency - adds the fence as a job dependency
>    * @job: scheduler job to add the dependencies to
> @@ -878,10 +910,12 @@ int drm_sched_job_add_dependency(struct drm_sched_job *job,
>   	 * engines involved, rather than the number of BOs.
>   	 */
>   	xa_for_each(&job->dependencies, index, entry) {
> -		if (entry->context != fence->context)
> +		bool signaled = dma_fence_is_signaled(entry);

I so dislike dma_fence_is_signaled() due it leaking the fence signaling 
annotations to unsuspecting callers. I hope it doesn't mark someone's 
lock/mutex as the critical signalling path..

Regards,

Tvrtko

> +
> +		if (!signaled && entry->context != fence->context)
>   			continue;
>   
> -		if (dma_fence_is_later(fence, entry)) {
> +		if (signaled || dma_fence_is_later(fence, entry)) {
>   			dma_fence_put(entry);
>   			xa_store(&job->dependencies, index, fence, GFP_KERNEL);
>   		} else {
> @@ -890,7 +924,8 @@ int drm_sched_job_add_dependency(struct drm_sched_job *job,
>   		return 0;
>   	}
>   
> -	ret = xa_alloc(&job->dependencies, &id, fence, xa_limit_32b, GFP_KERNEL);
> +	ret = xa_alloc(&job->dependencies, &id, fence, xa_limit_32b,
> +		       GFP_KERNEL);
>   	if (ret != 0)
>   		dma_fence_put(fence);
>   
> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> index 1a7e377d4cbb..916e820b27ff 100644
> --- a/include/drm/gpu_scheduler.h
> +++ b/include/drm/gpu_scheduler.h
> @@ -632,6 +632,8 @@ int drm_sched_job_init(struct drm_sched_job *job,
>   		       u32 credits, void *owner);
>   void drm_sched_job_arm(struct drm_sched_job *job);
>   void drm_sched_entity_push_job(struct drm_sched_job *sched_job);
> +int drm_sched_job_prealloc_dependency_slots(struct drm_sched_job *job,
> +					    unsigned int num_deps);
>   int drm_sched_job_add_dependency(struct drm_sched_job *job,
>   				 struct dma_fence *fence);
>   int drm_sched_job_add_syncobj_dependency(struct drm_sched_job *job,