Re: [PATCH v7 01/15] dma-buf/dma-fence: Add deadline awareness

[Pekka Paalanen]

On Mon, 27 Feb 2023 11:35:07 -0800
Rob Clark <robdclark@xxxxxxxxx> wrote:

> From: Rob Clark <robdclark@xxxxxxxxxxxx>
> 
> 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
> 
> Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> Reviewed-by: Christian König <christian.koenig@xxxxxxx>
> ---
>  Documentation/driver-api/dma-buf.rst |  6 +++
>  drivers/dma-buf/dma-fence.c          | 59 ++++++++++++++++++++++++++++
>  include/linux/dma-fence.h            | 20 ++++++++++
>  3 files changed, 85 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..e103e821d993 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 single to

Hi Rob,

s/single/signal/ ?

> + *   the GPU's devfreq to reduce frequency, when in fact the opposite is what is
> + *   needed.
> + *
> + * 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).

As "current time" not a special value, but just an absolute timestamp
like any other, deadlines already in the past must also be accepted?

> + *
> + * Multiple deadlines may be set on a given fence, even in parallel.  See the
> + * documentation for &dma_fence_ops.set_deadline.
> + *
> + * The deadline hint is just that, a hint.  The driver that created the fence
> + * may react by increasing frequency, making different scheduling choices, etc.
> + * Or doing nothing at all.
> + */

Yes! Thank you for writing this! Well explained.

> +
> +/**
> + * dma_fence_set_deadline - set desired fence-wait deadline hint
> + * @fence:    the fence that is to be waited on
> + * @deadline: the time by which the waiter hopes for the fence to be
> + *            signaled
> + *
> + * Give the fence signaler a hint about an upcoming deadline, such as
> + * vblank, by which point the waiter would prefer the fence to be
> + * signaled by.  This is intended to give feedback to the fence signaler
> + * to aid in power management decisions, such as boosting GPU frequency
> + * if a periodic vblank deadline is approaching but the fence is not
> + * yet signaled..
> + */
> +void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline)
> +{
> +	if (fence->ops->set_deadline && !dma_fence_is_signaled(fence))
> +		fence->ops->set_deadline(fence, deadline);
> +}
> +EXPORT_SYMBOL(dma_fence_set_deadline);
> +
>  /**
>   * dma_fence_describe - Dump fence describtion into seq_file
>   * @fence: the 6fence to describe
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 775cdc0b4f24..87c0d846dbb4 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -257,6 +257,24 @@ struct dma_fence_ops {
>  	 */
>  	void (*timeline_value_str)(struct dma_fence *fence,
>  				   char *str, int size);
> +
> +	/**
> +	 * @set_deadline:
> +	 *
> +	 * Callback to allow a fence waiter to inform the fence signaler of
> +	 * an upcoming deadline, such as vblank, by which point the waiter
> +	 * would prefer the fence to be signaled by.  This is intended to
> +	 * give feedback to the fence signaler to aid in power management
> +	 * decisions, such as boosting GPU frequency.
> +	 *
> +	 * This is called without &dma_fence.lock held, it can be called
> +	 * multiple times and from any context.  Locking is up to the callee
> +	 * if it has some state to manage.  If multiple deadlines are set,
> +	 * the expectation is to track the soonest one.
> +	 *
> +	 * This callback is optional.
> +	 */
> +	void (*set_deadline)(struct dma_fence *fence, ktime_t deadline);
>  };
>  
>  void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
> @@ -583,6 +601,8 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr)
>  	return ret < 0 ? ret : 0;
>  }
>  
> +void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline);
> +
>  struct dma_fence *dma_fence_get_stub(void);
>  struct dma_fence *dma_fence_allocate_private_stub(void);
>  u64 dma_fence_context_alloc(unsigned num);

This is exactly what I wanted to see. Already
Acked-by: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx>


Thanks,
pq

Attachment: pgpZHEgrtMf3y.pgp
Description: OpenPGP digital signature