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@xxxxxxxxxxxxxxx> wrote:
From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
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@xxxxxxx>
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
