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

[Christian König]

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.

Regards,
Christian.

> -Chris
>