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

[Daniel Vetter]

On Fri, Sep 23, 2016 at 07:59:44PM +0200, Christian König wrote:
> Am 23.09.2016 um 17:20 schrieb 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.
> > Oh, it also means that
> > 
> > commit fb8b7d2b9d80e1e71f379e57355936bd2b024be9
> > Author: Jammy Zhou <Jammy.Zhou at amd.com>
> > Date:   Wed Jan 21 18:35:47 2015 +0800
> > 
> >      reservation: wait only with non-zero timeout specified (v3)
> >      When the timeout value passed to reservation_object_wait_timeout_rcu
> >      is zero, no wait should be done if the fences are not signaled.
> >      Return '1' for idle and '0' for busy if the specified timeout is '0'
> >      to keep consistent with the case of non-zero timeout.
> >      v2: call fence_put if not signaled in the case of timeout==0
> >      v3: switch to reservation_object_test_signaled_rcu
> >      Signed-off-by: Jammy Zhou <Jammy.Zhou at amd.com>
> >      Reviewed-by: Christian König <christian.koenig at amd.com>
> >      Reviewed-by: Alex Deucher <alexander.deucher at amd.com>
> >      Reviewed-By: Maarten Lankhorst <maarten.lankhorst at canonical.com>
> >      Signed-off-by: Sumit Semwal <sumit.semwal at linaro.org>
> > 
> > is wrong. And reservation_object_test_signaled_rcu() is unreliable.
> 
> Ups indeed, that patch is wrong as well.
> 
> I suggest that we just enable the signaling in this case as well.

Will you/Zhou take care of this corner case? Just so I can't forget about
it ;-)

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch