[Intel-gfx] [RFC v2 0/5] Waitboost drm syncobj waits

[Tvrtko Ursulin]

On 16/02/2023 18:19, Rodrigo Vivi wrote:
> On Tue, Feb 14, 2023 at 11:14:00AM -0800, Rob Clark wrote:
>> On Fri, Feb 10, 2023 at 5:07 AM Tvrtko Ursulin
>> <tvrtko.ursulin at linux.intel.com> wrote:
>>>
>>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>>
>>> In i915 we have this concept of "wait boosting" where we give a priority boost
>>> for instance to fences which are actively waited upon from userspace. This has
>>> it's pros and cons and can certainly be discussed at lenght. However fact is
>>> some workloads really like it.
>>>
>>> Problem is that with the arrival of drm syncobj and a new userspace waiting
>>> entry point it added, the waitboost mechanism was bypassed. Hence I cooked up
>>> this mini series really (really) quickly to see if some discussion can be had.
>>>
>>> It adds a concept of "wait count" to dma fence, which is incremented for every
>>> explicit dma_fence_enable_sw_signaling and dma_fence_add_wait_callback (like
>>> dma_fence_add_callback but from explicit/userspace wait paths).
>>
>> I was thinking about a similar thing, but in the context of dma_fence
>> (or rather sync_file) fd poll()ing.  How does the kernel differentiate
>> between "housekeeping" poll()ers that don't want to trigger boost but
>> simply know when to do cleanup, and waiters who are waiting with some
>> urgency.  I think we could use EPOLLPRI for this purpose.
>>
>> Not sure how that translates to waits via the syncobj.  But I think we
>> want to let userspace give some hint about urgent vs housekeeping
>> waits.
> 
> Should the hint be on the waits, or should the hints be on the executed
> context?
> 
> In the end we need some way to quickly ramp-up the frequency to avoid
> the execution bubbles.
> 
> waitboost is trying to guess that, but in some cases it guess wrong
> and waste power.

Do we have a list of workloads which shows who benefits and who loses 
from the current implementation of waitboost?
> btw, this is something that other drivers might need:
> 
> https://gitlab.freedesktop.org/drm/amd/-/issues/1500#note_825883
> Cc: Alex Deucher <alexander.deucher at amd.com>

I have several issues with the context hint if it would directly 
influence frequency selection in the "more power" direction.

First of all, assume a context hint would replace the waitboost. Which 
applications would need to set it to restore the lost performance and 
how would they set it?

Then I don't even think userspace necessarily knows. Think of a layer 
like OpenCL. It doesn't really know in advance the profile of 
submissions vs waits. It depends on the CPU vs GPU speed, so hardware 
generation, and the actual size of the workload which can be influenced 
by the application (or user) and not the library.

The approach also lends itself well for the "arms race" where every 
application can say "Me me me, I am the most important workload there is!".

The last concern is for me shared with the proposal to expose deadlines 
or high priority waits as explicit uapi knobs. Both come under the "what 
application told us it will do" category vs what it actually does. So I 
think it is slightly weaker than basing decisions of waits.

The current waitboost is a bit detached from that problem because when 
we waitboost for flips we _know_ it is an actual framebuffer in the flip 
chain. When we waitboost for waits we also know someone is waiting. We 
are not trusting userspace telling us this will be a buffer in the flip 
chain or that this is a context which will have a certain duty-cycle.

But yes, even if the input is truthful, latter is still only a 
heuristics because nothing says all waits are important. AFAIU it just 
happened to work well in the past.

I do understand I am effectively arguing for more heuristics, which may 
sound a bit against the common wisdom. This is because in general I 
think the logic to do the right thing, be it in the driver or in the 
firmware, can work best if it has a holistic view. Simply put it needs 
to have more inputs to the decisions it is making.

That is what my series is proposing - adding a common signal of "someone 
in userspace is waiting". What happens with that signal needs not be 
defined (promised) in the uapi contract.

Say you route it to SLPC logic. It doesn't need to do with it what 
legacy i915 is doing today. It just needs to do something which works 
best for majority of workloads. It can even ignore it if that works for it.

Finally, back to the immediate problem is when people replace the OpenCL 
NEO driver with clvk that performance tanks. Because former does waits 
using i915 specific ioctls and so triggers waitboost, latter waits on 
syncobj so no waitboost and performance is bad. What short term solution 
can we come up with? Or we say to not use clvk? I also wonder if other 
Vulkan based stuff is perhaps missing those easy performance gains..

Perhaps strictly speaking Rob's and mine proposal are not mutually 
exclusive. Yes I could piggy back on his with an "immediate deadline for 
waits" idea, but they could also be separate concepts if we concluded 
"someone is waiting" signal is useful to have. Or it takes to long to 
upstream the full deadline idea.

Regards,

Tvrtko

>>
>> Also, on a related topic: https://lwn.net/Articles/868468/
>>
>> BR,
>> -R
>>
>>> Individual drivers can then inspect this via dma_fence_wait_count() and decide
>>> to wait boost the waits on such fences.
>>>
>>> Again, quickly put together and smoke tested only - no guarantees whatsoever and
>>> I will rely on interested parties to test and report if it even works or how
>>> well.
>>>
>>> v2:
>>>   * Small fixups based on CI feedback:
>>>      * Handle decrement correctly for already signalled case while adding callback.
>>>      * Remove i915 assert which was making sure struct i915_request does not grow.
>>>   * Split out the i915 patch into three separate functional changes.
>>>
>>> Tvrtko Ursulin (5):
>>>    dma-fence: Track explicit waiters
>>>    drm/syncobj: Mark syncobj waits as external waiters
>>>    drm/i915: Waitboost external waits
>>>    drm/i915: Mark waits as explicit
>>>    drm/i915: Wait boost requests waited upon by others
>>>
>>>   drivers/dma-buf/dma-fence.c               | 102 ++++++++++++++++------
>>>   drivers/gpu/drm/drm_syncobj.c             |   6 +-
>>>   drivers/gpu/drm/i915/gt/intel_engine_pm.c |   1 -
>>>   drivers/gpu/drm/i915/i915_request.c       |  13 ++-
>>>   include/linux/dma-fence.h                 |  14 +++
>>>   5 files changed, 101 insertions(+), 35 deletions(-)
>>>
>>> --
>>> 2.34.1
>>>