Re: [PATCH 1/6] drm/sched: Avoid memory leaks with cancel_job() callback

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

On 01/07/2025 14:21, Philipp Stanner wrote:
> Since its inception, the GPU scheduler can leak memory if the driver
> calls drm_sched_fini() while there are still jobs in flight.
> 
> The simplest way to solve this in a backwards compatible manner is by
> adding a new callback, drm_sched_backend_ops.cancel_job(), which
> instructs the driver to signal the hardware fence associated with the
> job. Afterwards, the scheduler can savely use the established free_job()
> callback for freeing the job.
> 
> Implement the new backend_ops callback cancel_job().
> 
> Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> Link: https://lore.kernel.org/dri-devel/20250418113211.69956-1-tvrtko.ursulin@igalia.com/
> Signed-off-by: Philipp Stanner <phasta@kernel.org>
> ---
>   drivers/gpu/drm/scheduler/sched_main.c | 34 ++++++++++++++++----------
>   include/drm/gpu_scheduler.h            | 18 ++++++++++++++
>   2 files changed, 39 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index c63543132f9d..1239954f5f7c 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -1353,6 +1353,18 @@ int drm_sched_init(struct drm_gpu_scheduler *sched, const struct drm_sched_init_
>   }
>   EXPORT_SYMBOL(drm_sched_init);
>   
> +static void drm_sched_cancel_remaining_jobs(struct drm_gpu_scheduler *sched)
> +{
> +	struct drm_sched_job *job, *tmp;
> +
> +	/* All other accessors are stopped. No locking necessary. */
> +	list_for_each_entry_safe_reverse(job, tmp, &sched->pending_list, list) {
> +		sched->ops->cancel_job(job);
> +		list_del(&job->list);

List_del is just for the warning in 3/6 I guess? You could in theory zap 
the whole list in one go and avoid the safe iterator. Not that it 
matters really so:

Acked-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>

Regards,

Tvrtko

> +		sched->ops->free_job(job);
> +	}
> +}
> +
>   /**
>    * drm_sched_fini - Destroy a gpu scheduler
>    *
> @@ -1360,19 +1372,11 @@ EXPORT_SYMBOL(drm_sched_init);
>    *
>    * Tears down and cleans up the scheduler.
>    *
> - * This stops submission of new jobs to the hardware through
> - * drm_sched_backend_ops.run_job(). Consequently, drm_sched_backend_ops.free_job()
> - * will not be called for all jobs still in drm_gpu_scheduler.pending_list.
> - * There is no solution for this currently. Thus, it is up to the driver to make
> - * sure that:
> - *
> - *  a) drm_sched_fini() is only called after for all submitted jobs
> - *     drm_sched_backend_ops.free_job() has been called or that
> - *  b) the jobs for which drm_sched_backend_ops.free_job() has not been called
> - *     after drm_sched_fini() ran are freed manually.
> - *
> - * FIXME: Take care of the above problem and prevent this function from leaking
> - * the jobs in drm_gpu_scheduler.pending_list under any circumstances.
> + * This stops submission of new jobs to the hardware through &struct
> + * drm_sched_backend_ops.run_job. If &struct drm_sched_backend_ops.cancel_job
> + * is implemented, all jobs will be canceled through it and afterwards cleaned
> + * up through &struct drm_sched_backend_ops.free_job. If cancel_job is not
> + * implemented, memory could leak.
>    */
>   void drm_sched_fini(struct drm_gpu_scheduler *sched)
>   {
> @@ -1402,6 +1406,10 @@ void drm_sched_fini(struct drm_gpu_scheduler *sched)
>   	/* Confirm no work left behind accessing device structures */
>   	cancel_delayed_work_sync(&sched->work_tdr);
>   
> +	/* Avoid memory leaks if supported by the driver. */
> +	if (sched->ops->cancel_job)
> +		drm_sched_cancel_remaining_jobs(sched);
> +
>   	if (sched->own_submit_wq)
>   		destroy_workqueue(sched->submit_wq);
>   	sched->ready = false;
> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> index e62a7214e052..190844370f48 100644
> --- a/include/drm/gpu_scheduler.h
> +++ b/include/drm/gpu_scheduler.h
> @@ -512,6 +512,24 @@ struct drm_sched_backend_ops {
>            * and it's time to clean it up.
>   	 */
>   	void (*free_job)(struct drm_sched_job *sched_job);
> +
> +	/**
> +	 * @cancel_job: Used by the scheduler to guarantee remaining jobs' fences
> +	 * get signaled in drm_sched_fini().
> +	 *
> +	 * Used by the scheduler to cancel all jobs that have not been executed
> +	 * with &struct drm_sched_backend_ops.run_job by the time
> +	 * drm_sched_fini() gets invoked.
> +	 *
> +	 * Drivers need to signal the passed job's hardware fence with an
> +	 * appropriate error code (e.g., -ECANCELED) in this callback. They
> +	 * must not free the job.
> +	 *
> +	 * The scheduler will only call this callback once it stopped calling
> +	 * all other callbacks forever, with the exception of &struct
> +	 * drm_sched_backend_ops.free_job.
> +	 */
> +	void (*cancel_job)(struct drm_sched_job *sched_job);
>   };
>   
>   /**