Re: [PATCH v4 12/13] drm/msm: Utilize gpu scheduler priorities

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

Hi Rob,

On 28/07/2021 02:06, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> The drm/scheduler provides additional prioritization on top of that
> provided by however many number of ringbuffers (each with their own
> priority level) is supported on a given generation.  Expose the
> additional levels of priority to userspace and map the userspace
> priority back to ring (first level of priority) and schedular priority
> (additional priority levels within the ring).
> 
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> Acked-by: Christian König <christian.koenig@amd.com>
> ---
>   drivers/gpu/drm/msm/adreno/adreno_gpu.c |  4 +-
>   drivers/gpu/drm/msm/msm_gem_submit.c    |  4 +-
>   drivers/gpu/drm/msm/msm_gpu.h           | 58 ++++++++++++++++++++++++-
>   drivers/gpu/drm/msm/msm_submitqueue.c   | 35 +++++++--------
>   include/uapi/drm/msm_drm.h              | 14 +++++-
>   5 files changed, 88 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/adreno/adreno_gpu.c b/drivers/gpu/drm/msm/adreno/adreno_gpu.c
> index bad4809b68ef..748665232d29 100644
> --- a/drivers/gpu/drm/msm/adreno/adreno_gpu.c
> +++ b/drivers/gpu/drm/msm/adreno/adreno_gpu.c
> @@ -261,8 +261,8 @@ int adreno_get_param(struct msm_gpu *gpu, uint32_t param, uint64_t *value)
>   			return ret;
>   		}
>   		return -EINVAL;
> -	case MSM_PARAM_NR_RINGS:
> -		*value = gpu->nr_rings;
> +	case MSM_PARAM_PRIORITIES:
> +		*value = gpu->nr_rings * NR_SCHED_PRIORITIES;
>   		return 0;
>   	case MSM_PARAM_PP_PGTABLE:
>   		*value = 0;
> diff --git a/drivers/gpu/drm/msm/msm_gem_submit.c b/drivers/gpu/drm/msm/msm_gem_submit.c
> index 450efe59abb5..c2ecec5b11c4 100644
> --- a/drivers/gpu/drm/msm/msm_gem_submit.c
> +++ b/drivers/gpu/drm/msm/msm_gem_submit.c
> @@ -59,7 +59,7 @@ static struct msm_gem_submit *submit_create(struct drm_device *dev,
>   	submit->gpu = gpu;
>   	submit->cmd = (void *)&submit->bos[nr_bos];
>   	submit->queue = queue;
> -	submit->ring = gpu->rb[queue->prio];
> +	submit->ring = gpu->rb[queue->ring_nr];
>   	submit->fault_dumped = false;
>   
>   	INIT_LIST_HEAD(&submit->node);
> @@ -749,7 +749,7 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
>   	/* Get a unique identifier for the submission for logging purposes */
>   	submitid = atomic_inc_return(&ident) - 1;
>   
> -	ring = gpu->rb[queue->prio];
> +	ring = gpu->rb[queue->ring_nr];
>   	trace_msm_gpu_submit(pid_nr(pid), ring->id, submitid,
>   		args->nr_bos, args->nr_cmds);
>   
> diff --git a/drivers/gpu/drm/msm/msm_gpu.h b/drivers/gpu/drm/msm/msm_gpu.h
> index b912cacaecc0..0e4b45bff2e6 100644
> --- a/drivers/gpu/drm/msm/msm_gpu.h
> +++ b/drivers/gpu/drm/msm/msm_gpu.h
> @@ -250,6 +250,59 @@ struct msm_gpu_perfcntr {
>   	const char *name;
>   };
>   
> +/*
> + * The number of priority levels provided by drm gpu scheduler.  The
> + * DRM_SCHED_PRIORITY_KERNEL priority level is treated specially in some
> + * cases, so we don't use it (no need for kernel generated jobs).
> + */
> +#define NR_SCHED_PRIORITIES (1 + DRM_SCHED_PRIORITY_HIGH - DRM_SCHED_PRIORITY_MIN)
> +
> +/**
> + * msm_gpu_convert_priority - Map userspace priority to ring # and sched priority
> + *
> + * @gpu:        the gpu instance
> + * @prio:       the userspace priority level
> + * @ring_nr:    [out] the ringbuffer the userspace priority maps to
> + * @sched_prio: [out] the gpu scheduler priority level which the userspace
> + *              priority maps to
> + *
> + * With drm/scheduler providing it's own level of prioritization, our total
> + * number of available priority levels is (nr_rings * NR_SCHED_PRIORITIES).
> + * Each ring is associated with it's own scheduler instance.  However, our
> + * UABI is that lower numerical values are higher priority.  So mapping the
> + * single userspace priority level into ring_nr and sched_prio takes some
> + * care.  The userspace provided priority (when a submitqueue is created)
> + * is mapped to ring nr and scheduler priority as such:
> + *
> + *   ring_nr    = userspace_prio / NR_SCHED_PRIORITIES
> + *   sched_prio = NR_SCHED_PRIORITIES -
> + *                (userspace_prio % NR_SCHED_PRIORITIES) - 1
> + *
> + * This allows generations without preemption (nr_rings==1) to have some
> + * amount of prioritization, and provides more priority levels for gens
> + * that do have preemption.

I am exploring how different drivers handle priority levels and this 
caught my eye.

Is the implication of the last paragraphs that on hw with nr_rings > 1, 
ring + 1 preempts ring? If so I am wondering does the "spreading" of 
user visible priorities by NR_SCHED_PRIORITIES creates a non-preemptable 
levels within every "bucket" or how does that work?

Regards,

Tvrtko

> + */
> +static inline int msm_gpu_convert_priority(struct msm_gpu *gpu, int prio,
> +		unsigned *ring_nr, enum drm_sched_priority *sched_prio)
> +{
> +	unsigned rn, sp;
> +
> +	rn = div_u64_rem(prio, NR_SCHED_PRIORITIES, &sp);
> +
> +	/* invert sched priority to map to higher-numeric-is-higher-
> +	 * priority convention
> +	 */
> +	sp = NR_SCHED_PRIORITIES - sp - 1;
> +
> +	if (rn >= gpu->nr_rings)
> +		return -EINVAL;
> +
> +	*ring_nr = rn;
> +	*sched_prio = sp;
> +
> +	return 0;
> +}
> +
>   /**
>    * A submitqueue is associated with a gl context or vk queue (or equiv)
>    * in userspace.
> @@ -257,7 +310,8 @@ struct msm_gpu_perfcntr {
>    * @id:        userspace id for the submitqueue, unique within the drm_file
>    * @flags:     userspace flags for the submitqueue, specified at creation
>    *             (currently unusued)
> - * @prio:      the submitqueue priority
> + * @ring_nr:   the ringbuffer used by this submitqueue, which is determined
> + *             by the submitqueue's priority
>    * @faults:    the number of GPU hangs associated with this submitqueue
>    * @ctx:       the per-drm_file context associated with the submitqueue (ie.
>    *             which set of pgtables do submits jobs associated with the
> @@ -272,7 +326,7 @@ struct msm_gpu_perfcntr {
>   struct msm_gpu_submitqueue {
>   	int id;
>   	u32 flags;
> -	u32 prio;
> +	u32 ring_nr;
>   	int faults;
>   	struct msm_file_private *ctx;
>   	struct list_head node;
> diff --git a/drivers/gpu/drm/msm/msm_submitqueue.c b/drivers/gpu/drm/msm/msm_submitqueue.c
> index 682ba2a7c0ec..32a55d81b58b 100644
> --- a/drivers/gpu/drm/msm/msm_submitqueue.c
> +++ b/drivers/gpu/drm/msm/msm_submitqueue.c
> @@ -68,6 +68,8 @@ int msm_submitqueue_create(struct drm_device *drm, struct msm_file_private *ctx,
>   	struct msm_gpu_submitqueue *queue;
>   	struct msm_ringbuffer *ring;
>   	struct drm_gpu_scheduler *sched;
> +	enum drm_sched_priority sched_prio;
> +	unsigned ring_nr;
>   	int ret;
>   
>   	if (!ctx)
> @@ -76,8 +78,9 @@ int msm_submitqueue_create(struct drm_device *drm, struct msm_file_private *ctx,
>   	if (!priv->gpu)
>   		return -ENODEV;
>   
> -	if (prio >= priv->gpu->nr_rings)
> -		return -EINVAL;
> +	ret = msm_gpu_convert_priority(priv->gpu, prio, &ring_nr, &sched_prio);
> +	if (ret)
> +		return ret;
>   
>   	queue = kzalloc(sizeof(*queue), GFP_KERNEL);
>   
> @@ -86,24 +89,13 @@ int msm_submitqueue_create(struct drm_device *drm, struct msm_file_private *ctx,
>   
>   	kref_init(&queue->ref);
>   	queue->flags = flags;
> -	queue->prio = prio;
> +	queue->ring_nr = ring_nr;
>   
> -	ring = priv->gpu->rb[prio];
> +	ring = priv->gpu->rb[ring_nr];
>   	sched = &ring->sched;
>   
> -	/*
> -	 * TODO we can allow more priorities than we have ringbuffers by
> -	 * mapping:
> -	 *
> -	 *    ring = prio / 3;
> -	 *    ent_prio = DRM_SCHED_PRIORITY_MIN + (prio % 3);
> -	 *
> -	 * Probably avoid using DRM_SCHED_PRIORITY_KERNEL as that is
> -	 * treated specially in places.
> -	 */
>   	ret = drm_sched_entity_init(&queue->entity,
> -			DRM_SCHED_PRIORITY_NORMAL,
> -			&sched, 1, NULL);
> +			sched_prio, &sched, 1, NULL);
>   	if (ret) {
>   		kfree(queue);
>   		return ret;
> @@ -134,16 +126,19 @@ int msm_submitqueue_create(struct drm_device *drm, struct msm_file_private *ctx,
>   int msm_submitqueue_init(struct drm_device *drm, struct msm_file_private *ctx)
>   {
>   	struct msm_drm_private *priv = drm->dev_private;
> -	int default_prio;
> +	int default_prio, max_priority;
>   
>   	if (!priv->gpu)
>   		return -ENODEV;
>   
> +	max_priority = (priv->gpu->nr_rings * NR_SCHED_PRIORITIES) - 1;
> +
>   	/*
> -	 * Select priority 2 as the "default priority" unless nr_rings is less
> -	 * than 2 and then pick the lowest priority
> +	 * Pick a medium priority level as default.  Lower numeric value is
> +	 * higher priority, so round-up to pick a priority that is not higher
> +	 * than the middle priority level.
>   	 */
> -	default_prio = clamp_t(uint32_t, 2, 0, priv->gpu->nr_rings - 1);
> +	default_prio = DIV_ROUND_UP(max_priority, 2);
>   
>   	INIT_LIST_HEAD(&ctx->submitqueues);
>   
> diff --git a/include/uapi/drm/msm_drm.h b/include/uapi/drm/msm_drm.h
> index f075851021c3..6b8fffc28a50 100644
> --- a/include/uapi/drm/msm_drm.h
> +++ b/include/uapi/drm/msm_drm.h
> @@ -73,11 +73,19 @@ struct drm_msm_timespec {
>   #define MSM_PARAM_MAX_FREQ   0x04
>   #define MSM_PARAM_TIMESTAMP  0x05
>   #define MSM_PARAM_GMEM_BASE  0x06
> -#define MSM_PARAM_NR_RINGS   0x07
> +#define MSM_PARAM_PRIORITIES 0x07  /* The # of priority levels */
>   #define MSM_PARAM_PP_PGTABLE 0x08  /* => 1 for per-process pagetables, else 0 */
>   #define MSM_PARAM_FAULTS     0x09
>   #define MSM_PARAM_SUSPENDS   0x0a
>   
> +/* For backwards compat.  The original support for preemption was based on
> + * a single ring per priority level so # of priority levels equals the #
> + * of rings.  With drm/scheduler providing additional levels of priority,
> + * the number of priorities is greater than the # of rings.  The param is
> + * renamed to better reflect this.
> + */
> +#define MSM_PARAM_NR_RINGS   MSM_PARAM_PRIORITIES
> +
>   struct drm_msm_param {
>   	__u32 pipe;           /* in, MSM_PIPE_x */
>   	__u32 param;          /* in, MSM_PARAM_x */
> @@ -304,6 +312,10 @@ struct drm_msm_gem_madvise {
>   
>   #define MSM_SUBMITQUEUE_FLAGS (0)
>   
> +/*
> + * The submitqueue priority should be between 0 and MSM_PARAM_PRIORITIES-1,
> + * a lower numeric value is higher priority.
> + */
>   struct drm_msm_submitqueue {
>   	__u32 flags;   /* in, MSM_SUBMITQUEUE_x */
>   	__u32 prio;    /* in, Priority level */