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

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

On 25/05/2022 14:41, Rob Clark wrote:
> On Wed, May 25, 2022 at 2:46 AM Tvrtko Ursulin
> <tvrtko.ursulin@linux.intel.com> wrote:
>>
>>
>> On 24/05/2022 15:50, 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:
>>>>> 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]
>>>>
>>>> Ah yes, I figure it out from the comments but then confused myself when
>>>> writing the email.
>>>>
>>>>>> 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
>>>>
>>>> Hmm how? Before run_job() the jobs are not runnable, sitting in the
>>>> scheduler queues, right?
>>>
>>> I mean, if prio[0]+prio[1]+prio[2] map to a single ring, submit A on
>>> prio[1] could be executed after submit B on prio[2] provided that
>>> run_job(submitA) hasn't happened yet.  So I guess it isn't "really"
>>> preemption because the submit hasn't started running on the GPU yet.
>>> But rather just scheduling according to priority.
>>>
>>>>> point, you only have "bucket" level preemption, because
>>>>> NR_SCHED_PRIORITIES levels of priority get mapped to a single FIFO
>>>>> ringbuffer.
>>>>
>>>> Right, and you have one GPU with four rings, which means you expose 12
>>>> priority levels to userspace, did I get that right?
>>>
>>> Correct
>>>
>>>> If so how do you convey in the ABI that not all there priority levels
>>>> are equal? Like userspace can submit at prio 4 and expect prio 3 to
>>>> preempt, as would prio 2 preempt prio 3. While actual behaviour will not
>>>> match - 3 will not preempt 4.
>>>
>>> It isn't really exposed to userspace, but perhaps it should be..
>>> Userspace just knows that, to the extent possible, the kernel will try
>>> to execute prio 3 before prio 4.
>>>
>>>> Also, does your userspace stack (EGL/Vulkan) use the priorities? I had a
>>>> quick peek in Mesa but did not spot it - although I am not really at
>>>> home there yet so maybe I missed it.
>>>
>>> Yes, there is an EGL extension:
>>>
>>> https://www.khronos.org/registry/EGL/extensions/IMG/EGL_IMG_context_priority.txt
>>>
>>> It is pretty limited, it only exposes three priority levels.
>>
>> Right, is that wired up on msm? And if it is, or could be, how do/would
>> you map the three priority levels for GPUs which expose 3 priority
>> levels versus the one which exposes 12?
> 
> We don't yet, but probably should, expose a cap to indicate to
> userspace the # of hw rings vs # of levels of sched priority

What bothers me is the question of whether this setup provides a 
consistent benefit. Why would userspace use other than "real" (hardware) 
priority levels on chips where they are available?

For instance if you exposed 4 instead of 12 on a respective platform, 
would that be better or worse? Yes you could only map three directly 
drm/sched and one would have to be "fake". Like:

hw prio 0 -> drm/sched 2
hw prio 1 -> drm/sched 1
hw prio 2 -> drm/sched 0
hw prio 3 -> drm/sched 0

Not saying that's nice either. Perhaps the answer is that drm/sched 
needs more flexibility for instance if it wants to be widely used.

For instance in i915 uapi we have priority as int -1023 - +1023. And 
matching implementation on some platforms, until the new ones which are 
GuC firmware based, where we need to squash that to low/normal/high.

So thinking was drm/sched happens to align with GuC. But then we have 
your hw where it doesn't seem to.

Regards,

Tvrtko

>> Is it doable properly without leaking the fact drm/sched internal
>> implementation detail of three priority levels? Or if you went the other
>> way and only exposed up to max 3 levels, then you lose one priority
>> level your hardware suppose which is also not good.
>>
>> It is all quite interesting because your hardware is completely
>> different from ours in this respect. In our case i915 decides when to
>> preempt, hardware has no concept of priority (*).
> 
> It is really pretty much all in firmware.. a6xx is the first gen that
> could do actual (non-cooperative) preemption (but that isn't
> implemented yet in upstream driver)
> 
> BR,
> -R
> 
>> Regards,
>>
>> Tvrtko
>>
>> (*) Almost no concept of priority in hardware - we do have it on new
>> GPUs and only on a subset of engine classes where render and compute
>> share the EUs. But I think it's way different from Ardenos.