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

[Rob Clark <robdclark@gmail.com>]

On Tue, May 24, 2022 at 6:45 AM Tvrtko Ursulin
<tvrtko.ursulin@linux.intel.com> wrote:
>
> On 23/05/2022 23:53, Rob Clark wrote:
> >
> > 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..
>
> Is that one or multiple threads submitting jobs?

In this case multiple.. but I think it could also happen with a single
thread (provided it didn't stall on a fence, directly or indirectly,
from an earlier submit), because of how resume and actual job
submission happens from scheduler kthread.

> > 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?
>
> On the direct reclaim path when invoked from the submit ioctl? In i915
> we only shrink idle objects on direct reclaim and leave active ones for
> the swapper. It depends on how your locking looks like whether you could
> do them, whether there would be coupling of locks and fs-reclaim context.

I think the locking is more or less ok, although lockdep is unhappy
about one thing[1] which is I think a false warning (ie. not
recognizing that we'd already successfully acquired the obj lock via
trylock).  We can already reclaim idle bo's in this path.  But the
problem with a bunch of submits queued up in the scheduler, is that
they are already considered pinned and active.  So at some point we
need to sleep (hopefully interruptabley) until they are no longer
active, ie. to throttle userspace trying to shove in more submits
until some of the enqueued ones have a chance to run and complete.

BR,
-R

[1] https://gitlab.freedesktop.org/drm/msm/-/issues/14

> Regards,
>
> Tvrtko
>
> > 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 */