Re: [PATCH 13/13] drm/i915: Cache last IRQ seqno to reduce IRQ overhead

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 11/12/15 13:12, John.C.Harrison@Intel.com wrote:
> From: John Harrison <John.C.Harrison@Intel.com>
>
> The notify function can be called many times without the seqno
> changing. A large number of duplicates are to prevent races due to the
> requirement of not enabling interrupts until requested. However, when
> interrupts are enabled the IRQ handle can be called multiple times
> without the ring's seqno value changing. This patch reduces the
> overhead of these extra calls by caching the last processed seqno
> value and early exiting if it has not changed.
>
> v3: New patch for series.
>
> For: VIZ-5190
> Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
> ---
>   drivers/gpu/drm/i915/i915_gem.c         | 14 +++++++++++---
>   drivers/gpu/drm/i915/intel_ringbuffer.h |  1 +
>   2 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 279d79f..3c88678 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2457,6 +2457,8 @@ i915_gem_init_seqno(struct drm_device *dev, u32 seqno)
>
>   		for (j = 0; j < ARRAY_SIZE(ring->semaphore.sync_seqno); j++)
>   			ring->semaphore.sync_seqno[j] = 0;
> +
> +		ring->last_irq_seqno = 0;
>   	}
>
>   	return 0;
> @@ -2788,11 +2790,14 @@ void i915_gem_request_notify(struct intel_engine_cs *ring, bool fence_locked)
>   		return;
>   	}
>
> -	if (!fence_locked)
> -		spin_lock_irqsave(&ring->fence_lock, flags);
> -
>   	seqno = ring->get_seqno(ring, false);
>   	trace_i915_gem_request_notify(ring, seqno);
> +	if (seqno == ring->last_irq_seqno)
> +		return;
> +	ring->last_irq_seqno = seqno;

Hmmm.. do you want to make the check "seqno <= ring->last_irq_seqno" ?

Is there a possibility for some weird timing or caching issue where two 
callers get in and last_irq_seqno goes backwards? Not sure that it would 
cause a problem, but pattern is unusual and hard to understand for me.

Also check and the assignment would need to be under the spinlock I think.

> +
> +	if (!fence_locked)
> +		spin_lock_irqsave(&ring->fence_lock, flags);
>
>   	list_for_each_entry_safe(req, req_next, &ring->fence_signal_list, signal_link) {
>   		if (!req->cancelled) {
> @@ -3163,7 +3168,10 @@ static void i915_gem_reset_ring_cleanup(struct drm_i915_private *dev_priv,
>   	 * Tidy up anything left over. This includes a call to
>   	 * i915_gem_request_notify() which will make sure that any requests
>   	 * that were on the signal pending list get also cleaned up.
> +	 * NB: The seqno cache must be cleared otherwise the notify call will
> +	 * simply return immediately.
>   	 */
> +	ring->last_irq_seqno = 0;
>   	i915_gem_retire_requests_ring(ring);
>
>   	/* Having flushed all requests from all queues, we know that all
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index 9d09edb..1987abd 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -356,6 +356,7 @@ struct  intel_engine_cs {
>   	spinlock_t fence_lock;
>   	struct list_head fence_signal_list;
>   	struct list_head fence_unsignal_list;
> +	uint32_t last_irq_seqno;
>   };
>
>   bool intel_ring_initialized(struct intel_engine_cs *ring);
>

Regards,

Tvrtko
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