[PATCH v10 01/15] dma-buf/dma-fence: Add deadline awareness

Fri Mar 17 09:09:21 UTC 2023

On Thu, 16 Mar 2023 23:22:24 +0100
Sebastian Wick <sebastian.wick at redhat.com> wrote:

> On Thu, Mar 16, 2023 at 5:29 PM Rob Clark <robdclark at gmail.com> wrote:
> >
> > On Thu, Mar 16, 2023 at 2:26 AM Jonas Ådahl <jadahl at gmail.com> wrote:  
> > >
> > > On Wed, Mar 15, 2023 at 09:19:49AM -0700, Rob Clark wrote:  
> > > > On Wed, Mar 15, 2023 at 6:53 AM Jonas Ådahl <jadahl at gmail.com> wrote:  
> > > > >
> > > > > On Fri, Mar 10, 2023 at 09:38:18AM -0800, Rob Clark wrote:  
> > > > > > On Fri, Mar 10, 2023 at 7:45 AM Jonas Ådahl <jadahl at gmail.com> wrote:  
> > > > > > >
> > > > > > > On Wed, Mar 08, 2023 at 07:52:52AM -0800, Rob Clark wrote:  
> > > > > > > > From: Rob Clark <robdclark at chromium.org>
> > > > > > > >
> > > > > > > > Add a way to hint to the fence signaler of an upcoming deadline, such as
> > > > > > > > vblank, which the fence waiter would prefer not to miss.  This is to aid
> > > > > > > > the fence signaler in making power management decisions, like boosting
> > > > > > > > frequency as the deadline approaches and awareness of missing deadlines
> > > > > > > > so that can be factored in to the frequency scaling.
> > > > > > > >
> > > > > > > > v2: Drop dma_fence::deadline and related logic to filter duplicate
> > > > > > > >     deadlines, to avoid increasing dma_fence size.  The fence-context
> > > > > > > >     implementation will need similar logic to track deadlines of all
> > > > > > > >     the fences on the same timeline.  [ckoenig]
> > > > > > > > v3: Clarify locking wrt. set_deadline callback
> > > > > > > > v4: Clarify in docs comment that this is a hint
> > > > > > > > v5: Drop DMA_FENCE_FLAG_HAS_DEADLINE_BIT.
> > > > > > > > v6: More docs
> > > > > > > > v7: Fix typo, clarify past deadlines
> > > > > > > >
> > > > > > > > Signed-off-by: Rob Clark <robdclark at chromium.org>
> > > > > > > > Reviewed-by: Christian König <christian.koenig at amd.com>
> > > > > > > > Acked-by: Pekka Paalanen <pekka.paalanen at collabora.com>
> > > > > > > > Reviewed-by: Bagas Sanjaya <bagasdotme at gmail.com>
> > > > > > > > ---  
> > > > > > >
> > > > > > > Hi Rob!
> > > > > > >  
> > > > > > > >  Documentation/driver-api/dma-buf.rst |  6 +++
> > > > > > > >  drivers/dma-buf/dma-fence.c          | 59 ++++++++++++++++++++++++++++
> > > > > > > >  include/linux/dma-fence.h            | 22 +++++++++++
> > > > > > > >  3 files changed, 87 insertions(+)
> > > > > > > >
> > > > > > > > diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
> > > > > > > > index 622b8156d212..183e480d8cea 100644
> > > > > > > > --- a/Documentation/driver-api/dma-buf.rst
> > > > > > > > +++ b/Documentation/driver-api/dma-buf.rst
> > > > > > > > @@ -164,6 +164,12 @@ DMA Fence Signalling Annotations
> > > > > > > >  .. kernel-doc:: drivers/dma-buf/dma-fence.c
> > > > > > > >     :doc: fence signalling annotation
> > > > > > > >
> > > > > > > > +DMA Fence Deadline Hints
> > > > > > > > +~~~~~~~~~~~~~~~~~~~~~~~~
> > > > > > > > +
> > > > > > > > +.. kernel-doc:: drivers/dma-buf/dma-fence.c
> > > > > > > > +   :doc: deadline hints
> > > > > > > > +
> > > > > > > >  DMA Fences Functions Reference
> > > > > > > >  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > > > > >
> > > > > > > > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> > > > > > > > index 0de0482cd36e..f177c56269bb 100644
> > > > > > > > --- a/drivers/dma-buf/dma-fence.c
> > > > > > > > +++ b/drivers/dma-buf/dma-fence.c
> > > > > > > > @@ -912,6 +912,65 @@ dma_fence_wait_any_timeout(struct dma_fence **fences, uint32_t count,
> > > > > > > >  }
> > > > > > > >  EXPORT_SYMBOL(dma_fence_wait_any_timeout);
> > > > > > > >
> > > > > > > > +/**
> > > > > > > > + * DOC: deadline hints
> > > > > > > > + *
> > > > > > > > + * In an ideal world, it would be possible to pipeline a workload sufficiently
> > > > > > > > + * that a utilization based device frequency governor could arrive at a minimum
> > > > > > > > + * frequency that meets the requirements of the use-case, in order to minimize
> > > > > > > > + * power consumption.  But in the real world there are many workloads which
> > > > > > > > + * defy this ideal.  For example, but not limited to:
> > > > > > > > + *
> > > > > > > > + * * Workloads that ping-pong between device and CPU, with alternating periods
> > > > > > > > + *   of CPU waiting for device, and device waiting on CPU.  This can result in
> > > > > > > > + *   devfreq and cpufreq seeing idle time in their respective domains and in
> > > > > > > > + *   result reduce frequency.
> > > > > > > > + *
> > > > > > > > + * * Workloads that interact with a periodic time based deadline, such as double
> > > > > > > > + *   buffered GPU rendering vs vblank sync'd page flipping.  In this scenario,
> > > > > > > > + *   missing a vblank deadline results in an *increase* in idle time on the GPU
> > > > > > > > + *   (since it has to wait an additional vblank period), sending a signal to
> > > > > > > > + *   the GPU's devfreq to reduce frequency, when in fact the opposite is what is
> > > > > > > > + *   needed.  
> > > > > > >
> > > > > > > This is the use case I'd like to get some better understanding about how
> > > > > > > this series intends to work, as the problematic scheduling behavior
> > > > > > > triggered by missed deadlines has plagued compositing display servers
> > > > > > > for a long time.
> > > > > > >
> > > > > > > I apologize, I'm not a GPU driver developer, nor an OpenGL driver
> > > > > > > developer, so I will need some hand holding when it comes to
> > > > > > > understanding exactly what piece of software is responsible for
> > > > > > > communicating what piece of information.
> > > > > > >  
> > > > > > > > + *
> > > > > > > > + * To this end, deadline hint(s) can be set on a &dma_fence via &dma_fence_set_deadline.
> > > > > > > > + * The deadline hint provides a way for the waiting driver, or userspace, to
> > > > > > > > + * convey an appropriate sense of urgency to the signaling driver.
> > > > > > > > + *
> > > > > > > > + * A deadline hint is given in absolute ktime (CLOCK_MONOTONIC for userspace
> > > > > > > > + * facing APIs).  The time could either be some point in the future (such as
> > > > > > > > + * the vblank based deadline for page-flipping, or the start of a compositor's
> > > > > > > > + * composition cycle), or the current time to indicate an immediate deadline
> > > > > > > > + * hint (Ie. forward progress cannot be made until this fence is signaled).  
> > > > > > >
> > > > > > > Is it guaranteed that a GPU driver will use the actual start of the
> > > > > > > vblank as the effective deadline? I have some memories of seing
> > > > > > > something about vblank evasion browsing driver code, which I might have
> > > > > > > misunderstood, but I have yet to find whether this is something
> > > > > > > userspace can actually expect to be something it can rely on.  
> > > > > >
> > > > > > I guess you mean s/GPU driver/display driver/ ?  It makes things more
> > > > > > clear if we talk about them separately even if they happen to be the
> > > > > > same device.  
> > > > >
> > > > > Sure, sorry about being unclear about that.
> > > > >  
> > > > > >
> > > > > > Assuming that is what you mean, nothing strongly defines what the
> > > > > > deadline is.  In practice there is probably some buffering in the
> > > > > > display controller.  For ex, block based (including bandwidth
> > > > > > compressed) formats, you need to buffer up a row of blocks to
> > > > > > efficiently linearize for scanout.  So you probably need to latch some
> > > > > > time before you start sending pixel data to the display.  But details
> > > > > > like this are heavily implementation dependent.  I think the most
> > > > > > reasonable thing to target is start of vblank.  
> > > > >
> > > > > The driver exposing those details would be quite useful for userspace
> > > > > though, so that it can delay committing updates to late, but not too
> > > > > late. Setting a deadline to be the vblank seems easy enough, but it
> > > > > isn't enough for scheduling the actual commit.  
> > > >
> > > > I'm not entirely sure how that would even work.. but OTOH I think you
> > > > are talking about something on the order of 100us?  But that is a bit
> > > > of another topic.  
> > >
> > > Yes, something like that. But yea, it's not really related. Scheduling
> > > commits closer to the deadline has more complex behavior than that too,
> > > e.g. the need for real time scheduling, and knowing how long it usually
> > > takes to create and commit and for the kernel to process.  
> 
> Vblank can be really long, especially with VRR where the additional
> time you get to finish the frame comes from making vblank longer.
> Using the start of vblank as a deadline makes VRR useless. It really
> would be nice to have some feedback about the actual deadline from the
> kernel, maybe in `struct drm_event_vblank`.

Hi Sebastian,

why would deadline at vblank beginning not be exactly what you want
also with VRR?

Let's say the frame misses that deadline on a VRR system, but the frame
still completes and the flip makes it to the intended scanout cycle,
thanks to VRR giving it more time. Why should that miss not be
classified as a miss?

If it is classified as a miss, then GPU freq and whatnot will be
increased, which will increase the effective update rate of VRR.

If it is not classified as a miss, then GPU might not speed up, and you
end up with low update rate, even though you still don't skip scanout
cycles thanks to VRR.

I guess if you actually *want* VRR to run at a low rate in order to
keep the GPU in a lower power demand, you could compute your "late
deadline" from the minimum rate VRR timings, subtracting the vblank
period from the estimated next flip completion timestamp?

Thanks,
pq
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