Re: [PATCH] drm/i915: Record the tail at each request and use it to estimate the head

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Feb 08, 2012 at 06:06:30PM +0000, Chris Wilson wrote:
> By recording the location of every request in the ringbuffer, we know
> that in order to retire the request the GPU must have finished reading
> it and so the GPU head is now beyond the tail of the request. We can
> therefore provide a conservative estimate of where the GPU is reading
> from in order to avoid having to read back the ring buffer registers
> when polling for space upon starting a new write into the ringbuffer.
> 
> A secondary effect is that this allows us to convert
> intel_ring_buffer_wait() to use i915_wait_request() and so consolidate
> upon the single function to handle the complicated task of waiting upon
> the GPU. A necessary precaution is that we need to make that wait
> uninterruptible to match the existing conditions as all the callers of
> intel_ring_begin() have not been audited to handle ERESTARTSYS
> correctly.
> 
> By using a conservative estimate for the head, and always processing all
> outstanding requests first, we prevent a race condition between using
> the estimate and direct reads of I915_RING_HEAD which could result in
> the value of the head going backwards, and the tail overflowing once
> again. We are also careful to mark any request that we skip over in
> order to free space in ring as consumed which provides a
> self-consistency check.
> 
> Given sufficient abuse, such as a set of unthrottled GPU bound
> cairo-traces, avoiding the use of I915_RING_HEAD gives a 10-20% boost on
> Sandy Bridge (i5-2520m):
>   firefox-paintball  18927ms -> 15646ms: 1.21x speedup
>   firefox-fishtank   12563ms -> 11278ms: 1.11x speedup
> which is a mild consolation for the performance those traces achieved from
> exploiting the buggy autoreported head.
> 
> v2: Add a few more comments and make request->tail a conservative
> estimate as suggested by Daniel Vetter.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48