On 02/05/2024 14:07, Christian König wrote:
Am 01.05.24 um 15:27 schrieb Tvrtko Ursulin:
Hi Alex,
On 30/04/2024 19:32, Alex Deucher wrote:
On Tue, Apr 30, 2024 at 1:27 PM Tvrtko Ursulin <tursu...@igalia.com>
wrote:
From: Tvrtko Ursulin <tvrtko.ursu...@igalia.com>
I have noticed AMD GPUs can have more than one "engine" (ring?) of
the same type
but amdgpu is not reporting that in fdinfo using the capacity engine
tag.
This series is therefore an attempt to improve that, but only an RFC
since it is
quite likely I got stuff wrong on the first attempt. Or if not wrong
it may not
be very beneficial in AMDs case.
So I tried to figure out how to count and store the number of
instances of an
"engine" type and spotted that could perhaps be used in more than
one place in
the driver. I was more than a little bit confused by the ip_instance
and uapi
rings, then how rings are selected to context entities internally.
Anyway..
hopefully it is a simple enough series to easily spot any such large
misses.
End result should be that, assuming two "engine" instances, one
fully loaded and
one idle will only report client using 50% of that engine type.
That would only be true if there are multiple instantiations of the IP
on the chip which in most cases is not true. In most cases there is
one instance of the IP that can be fed from multiple rings. E.g. for
graphics and compute, all of the rings ultimately feed into the same
compute units on the chip. So if you have a gfx ring and a compute
rings, you can schedule work to them asynchronously, but ultimately
whether they execute serially or in parallel depends on the actual
shader code in the command buffers and the extent to which it can
utilize the available compute units in the shader cores.
This is the same as with Intel/i915. Fdinfo is not intended to provide
utilisation of EUs and such, just how busy are the "entities" kernel
submits to. So doing something like in this series would make the
reporting more similar between the two drivers.
I think both the 0-800% or 0-100% range (taking 8 ring compute as an
example) can be misleading for different workloads. Neither <800% in
the former means one can send more work and same for <100% in the latter.
Yeah, I think that's what Alex tries to describe. By using 8 compute
rings your 800% load is actually incorrect and quite misleading.
Background is that those 8 compute rings won't be active all at the same
time, but rather waiting on each other for resources.
But this "waiting" is unfortunately considered execution time since the
used approach is actually not really capable of separating waiting and
execution time.
Right, so 800% is what gputop could be suggesting today, by the virtue 8
context/clients can each use 100% if they only use a subset of compute
units. I was proposing to expose the capacity in fdinfo so it can be
scaled down and then dicussing how both situation have pros and cons.
There is also a parallel with the CPU world here and hyper threading,
if not wider, where "What does 100% actually mean?" is also wishy-washy.
Also note that the reporting of actual time based values in fdinfo
would not changing with this series.
Of if you can guide me towards how to distinguish real vs fake
parallelism in HW IP blocks I could modify the series to only add
capacity tags where there are truly independent blocks. That would be
different from i915 though were I did not bother with that
distinction. (For reasons that assignment of for instance EUs to
compute "rings" (command streamers in i915) was supposed to be
possible to re-configure on the fly. So it did not make sense to try
and be super smart in fdinfo.)
Well exactly that's the point we don't really have truly independent
blocks on AMD hardware.
There are things like independent SDMA instances, but those a meant to
be used like the first instance for uploads and the second for downloads
etc.. When you use both instances for the same job they will pretty much
limit each other because of a single resource.
So _never_ multiple instances of the same IP block? No video decode,
encode, anything?
As for the UAPI portion of this, we generally expose a limited number
of rings to user space and then we use the GPU scheduler to load
balance between all of the available rings of a type to try and
extract as much parallelism as we can.
The part I do not understand is the purpose of the ring argument in
for instance drm_amdgpu_cs_chunk_ib. It appears userspace can create
up to N scheduling entities using different ring id's, but internally
they can map to 1:N same scheduler instances (depending on IP type,
can be that each userspace ring maps to same N hw rings, or for rings
with no drm sched load balancing userspace ring also does not appear
to have a relation to the picked drm sched instance.).
So I neither understand how this ring is useful, or how it does not
create a problem for IP types which use drm_sched_pick_best. It
appears even if userspace created two scheduling entities with
different ring ids they could randomly map to same drm sched aka same
hw ring, no?
Yeah, that is correct. The multimedia instances have to use a "fixed"
load balancing because of lack of firmware support. That should have
been fixed by now but we never found time to actually validate it.
Gotcha.
Regarding the "ring" parameter in CS, that is basically just for
backward compatibility with older userspace. E.g. that we don't map all
SDMA jobs to the same instance when only once context is used.
I see. In that sense "limits" for compute in amdgpu_ctx_num_entities are
arbitrary, or related to some old userspace expectation?
Regards,
Tvrtko
Regards,
Christian.
Regards,
Tvrtko
Alex
Tvrtko Ursulin (5):
drm/amdgpu: Cache number of rings per hw ip type
drm/amdgpu: Use cached number of rings from the
AMDGPU_INFO_HW_IP_INFO
ioctl
drm/amdgpu: Skip not present rings in amdgpu_ctx_mgr_usage
drm/amdgpu: Show engine capacity in fdinfo
drm/amdgpu: Only show VRAM in fdinfo if it exists
drivers/gpu/drm/amd/amdgpu/amdgpu.h | 1 +
drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c | 3 ++
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c | 14 +++++
drivers/gpu/drm/amd/amdgpu/amdgpu_fdinfo.c | 39 +++++++++-----
drivers/gpu/drm/amd/amdgpu/amdgpu_kms.c | 62
+++-------------------
5 files changed, 49 insertions(+), 70 deletions(-)
--
2.44.0
