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

From: Tvrtko Ursulin
Date: Wed May 25 2022 - 05:46:59 EST


On 24/05/2022 15:50, Rob Clark wrote:
On Tue, May 24, 2022 at 6:45 AM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:


On 23/05/2022 23:53, Rob Clark wrote:
On Mon, May 23, 2022 at 7:45 AM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:


Hi Rob,

On 28/07/2021 02:06, Rob Clark wrote:
From: Rob Clark <robdclark@xxxxxxxxxxxx>

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@xxxxxxxxxxxx>
Acked-by: Christian König <christian.koenig@xxxxxxx>
---
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?

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 (*).

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.