[Intel-gfx] [PATCH 11/11] dma-buf: Do a fast lockless check for poll with timeout=0

[Chris Wilson]

On Fri, Sep 23, 2016 at 03:50:44PM +0200, Daniel Vetter wrote:
> On Mon, Aug 29, 2016 at 08:08:34AM +0100, Chris Wilson wrote:
> > Currently we install a callback for performing poll on a dma-buf,
> > irrespective of the timeout. This involves taking a spinlock, as well as
> > unnecessary work, and greatly reduces scaling of poll(.timeout=0) across
> > multiple threads.
> > 
> > We can query whether the poll will block prior to installing the
> > callback to make the busy-query fast.
> > 
> > Single thread: 60% faster
> > 8 threads on 4 (+4 HT) cores: 600% faster
> > 
> > Still not quite the perfect scaling we get with a native busy ioctl, but
> > poll(dmabuf) is faster due to the quicker lookup of the object and
> > avoiding drm_ioctl().
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Sumit Semwal <sumit.semwal at linaro.org>
> > Cc: linux-media at vger.kernel.org
> > Cc: dri-devel at lists.freedesktop.org
> > Cc: linaro-mm-sig at lists.linaro.org
> > Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> 
> Need to strike the r-b here, since Christian König pointed out that
> objects won't magically switch signalling on.

The point being here that we don't even want to switch signaling on! :)

Christian's point was that not all fences guarantee forward progress
irrespective of whether signaling is enabled or not, and fences are not
required to guarantee forward progress without signaling even if they
provide an ops->signaled().
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre