Re: [Linaro-mm-sig] [RFC 1/3] dma-fence: Add boost fence op
From: Christian König
Date: Fri May 21 2021 - 03:44:12 EST
Am 20.05.21 um 19:08 schrieb Daniel Vetter:
[SNIP]
AH! So we are basically telling the fence backend that we have just
missed an event we waited for.
So what we want to know is how long the frontend wanted to wait instead
of how long the backend took for rendering.
tbh I'm not sure the timestamp matters at all. What we do in i915 is
boost quite aggressively, and then let the usual clock tuning wittle
it down if we overshot. Plus soom cool-down to prevent
abuse/continuous boosting. I think we also differentiate between
display boost and userspace waits.
I was not thinking about time stamps here, but more like which
information we need at which place.
On the display side we also wait until the vblank has passed we aimed
for (atm always the next, we don't have target_frame support like
amdgpu), to avoid boosting when there's no point.
So boosting right when you've missed your frame (not what Rob implements
currently, but fixable) is the right semantics.
The other issue is that for cpu waits, we want to differentiate from fence
waits that userspace does intentially (e.g. wait ioctl) and waits that
random other things are doing within the kernel to keep track of progress.
For the former we know that userspace is stuck waiting for the gpu, and we
probably want to boost. For the latter we most definitely do _not_ want to
boost.
Otoh I do agree with you that the current api is a bit awkward, so perhaps
we do need a dma_fence_userspace_wait wrapper which boosts automatically
after a bit. And similarly perhaps a drm_vblank_dma_fence_wait, where you
give it a vblank target, and if the fence isn't signalled by then, we kick
it real hard.
Yeah, something like an use case driven API would be nice to have.
For this particular case I suggest that we somehow extend the enable
signaling callback.
But otherwise yes this is absolutely a thing that matters a ton. If you
look at Matt Brost's scheduler rfc, there's also a line item in there
about adding this kind of boosting to drm/scheduler.
BTW: I still can't see this in my inbox.
You've replied already:
https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F20210518235830.133834-1-matthew.brost%40intel.com%2F&data=04%7C01%7Cchristian.koenig%40amd.com%7Ce4f3688b832842c4236e08d91bb1e148%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637571273080820910%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=uk3Gs%2FW42BDqMuMJtujcAH5GvN8mOlDnmywK8x1I%2F0k%3D&reserved=0
Yeah, but doesn't that also require some changes to the DRM scheduler?
I was expecting that this is a bit more than just two patches.
Christian.
It's just the big picture plan of what areas we're all trying to
tackle with some why, so that everyone knows what's coming in the next
half year at least. Probably longer until this is all sorted. I think
Matt has some poc hacked-up pile, but nothing really to show.
-Daniel
Do you have a link?
Christian.
-Daniel
Regards,
Christian.
BR,
-R
Thanks,
Christian.
BR,
-R
Christian.
Am 19.05.21 um 20:38 schrieb Rob Clark:
From: Rob Clark <robdclark@xxxxxxxxxxxx>
Add a way to hint to the fence signaler that a fence waiter has missed a
deadline waiting on the fence.
In some cases, missing a vblank can result in lower gpu utilization,
when really we want to go in the opposite direction and boost gpu freq.
The boost callback gives some feedback to the fence signaler that we
are missing deadlines, so it can take this into account in it's freq/
utilization calculations.
Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
---
include/linux/dma-fence.h | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index 9f12efaaa93a..172702521acc 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -231,6 +231,17 @@ struct dma_fence_ops {
signed long (*wait)(struct dma_fence *fence,
bool intr, signed long timeout);
+ /**
+ * @boost:
+ *
+ * Optional callback, to indicate that a fence waiter missed a deadline.
+ * This can serve as a signal that (if possible) whatever signals the
+ * fence should boost it's clocks.
+ *
+ * This can be called in any context that can call dma_fence_wait().
+ */
+ void (*boost)(struct dma_fence *fence);
+
/**
* @release:
*
@@ -586,6 +597,21 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr)
return ret < 0 ? ret : 0;
}
+/**
+ * dma_fence_boost - hint from waiter that it missed a deadline
+ *
+ * @fence: the fence that caused the missed deadline
+ *
+ * This function gives a hint from a fence waiter that a deadline was
+ * missed, so that the fence signaler can factor this in to device
+ * power state decisions
+ */
+static inline void dma_fence_boost(struct dma_fence *fence)
+{
+ if (fence->ops->boost)
+ fence->ops->boost(fence);
+}
+
struct dma_fence *dma_fence_get_stub(void);
u64 dma_fence_context_alloc(unsigned num);
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