[Intel-gfx] [PATCH 3/3] drm/i915: Micro-optimise hotpath through intel_ring_begin()

[Chris Wilson]

On Thu, May 04, 2017 at 03:11:45PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > Typically, there is space available within the ring and if not we have
> > to wait (by definition a slow path). Rearrange the code to reduce the
> > number of branches and stack size for the hotpath, accomodating a slight
> > growth for the wait.
> >
> > v2: Fix the new assert that packets are not larger than the actual ring.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/intel_ringbuffer.c | 63 +++++++++++++++++----------------
> >  1 file changed, 33 insertions(+), 30 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > index c46e5439d379..53123c1cfcc5 100644
> > --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> > @@ -1654,7 +1654,7 @@ static int ring_request_alloc(struct drm_i915_gem_request *request)
> >  	return 0;
> >  }
> >  
> > -static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
> > +static noinline int wait_for_space(struct drm_i915_gem_request *req, int bytes)
> >  {
> >  	struct intel_ring *ring = req->ring;
> >  	struct drm_i915_gem_request *target;
> > @@ -1702,49 +1702,52 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
> >  u32 *intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
> >  {
> >  	struct intel_ring *ring = req->ring;
> > -	int remain_actual = ring->size - ring->emit;
> > -	int remain_usable = ring->effective_size - ring->emit;
> > -	int bytes = num_dwords * sizeof(u32);
> > -	int total_bytes, wait_bytes;
> > -	bool need_wrap = false;
> > +	const unsigned int remain_usable = ring->effective_size - ring->emit;
> > +	const unsigned int bytes = num_dwords * sizeof(u32);
> > +	unsigned int need_wrap = 0;
> > +	unsigned int total_bytes;
> >  	u32 *cs;
> >  
> >  	total_bytes = bytes + req->reserved_space;
> > +	GEM_BUG_ON(total_bytes > ring->effective_size);
> >  
> > -	if (unlikely(bytes > remain_usable)) {
> > -		/*
> > -		 * Not enough space for the basic request. So need to flush
> > -		 * out the remainder and then wait for base + reserved.
> > -		 */
> > -		wait_bytes = remain_actual + total_bytes;
> > -		need_wrap = true;
> > -	} else if (unlikely(total_bytes > remain_usable)) {
> > -		/*
> > -		 * The base request will fit but the reserved space
> > -		 * falls off the end. So we don't need an immediate wrap
> > -		 * and only need to effectively wait for the reserved
> > -		 * size space from the start of ringbuffer.
> > -		 */
> > -		wait_bytes = remain_actual + req->reserved_space;
> > -	} else {
> > -		/* No wrapping required, just waiting. */
> > -		wait_bytes = total_bytes;
> > +	if (unlikely(total_bytes > remain_usable)) {
> > +		const int remain_actual = ring->size - ring->emit;
> > +
> > +		if (bytes > remain_usable) {
> > +			/*
> > +			 * Not enough space for the basic request. So need to
> > +			 * flush out the remainder and then wait for
> > +			 * base + reserved.
> > +			 */
> > +			total_bytes += remain_actual;
> > +			need_wrap = remain_actual | 1;
> 
> Your remain_actual should never reach zero. So in here
> forcing the lowest bit on, and later off, seems superfluous.

Why can't we fill up to the last byte with commands? remain_actual is
just (size - tail) and we don't force a wrap until emit crosses the
boundary (and not before). We hit remain_actual == 0 in practice.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre