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

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

On 24/05/2022 15:57, Rob Clark wrote:
> 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.

Odd I did not think trylock could trigger that. Looking at your code it 
indeed seems two trylocks. I am pretty sure we use the same trylock 
trick to avoid it. I am confused..

Otherwise if you can afford to sleep you can of course throttle 
organically via direct reclaim. Unless I am forgetting some key gotcha - 
it's been a while I've been active in this area.

Regards,

Tvrtko

> 
> 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 */