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

[Rob Clark <robdclark@chromium.org>]

On Mon, May 23, 2022 at 7:45 AM Tvrtko Ursulin
<tvrtko.ursulin@linux.intel.com> wrote:
>
>
> 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?

Other way around, at least from the uabi standpoint.  Ie. ring[0]
preempts ring[1]

> 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?

So, preemption is possible between any priority level before run_job()
gets called, which writes the job into the ringbuffer.  After that
point, you only have "bucket" level preemption, because
NR_SCHED_PRIORITIES levels of priority get mapped to a single FIFO
ringbuffer.

-----

btw, one fun (but unrelated) issue I'm hitting with scheduler... I'm
trying to add an igt test to stress shrinker/eviction, similar to the
existing tests/i915/gem_shrink.c.  But we hit an unfortunate
combination of circumstances:
1. Pinning memory happens in the synchronous part of the submit ioctl,
before enqueuing the job for the kthread to handle.
2. The first run_job() callback incurs a slight delay (~1.5ms) while
resuming the GPU
3. Because of that delay, userspace has a chance to queue up enough
more jobs to require locking/pinning more than the available system
RAM..

I'm not sure if we want a way to prevent userspace from getting *too*
far ahead of the kthread.  Or maybe at some point the shrinker should
sleep on non-idle buffers?

BR,
-R

>
> 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 */