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

[Daniel Vetter]

On Sun, Aug 28, 2016 at 05:37:47PM +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>

> ---
>  drivers/dma-buf/dma-buf.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index cf04d249a6a4..c7a7bc579941 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -156,6 +156,18 @@ static unsigned int dma_buf_poll(struct file *file, poll_table *poll)
>  	if (!events)
>  		return 0;
>  
> +	if (poll_does_not_wait(poll)) {
> +		if (events & POLLOUT &&
> +		    !reservation_object_test_signaled_rcu(resv, true))
> +			events &= ~(POLLOUT | POLLIN);
> +
> +		if (events & POLLIN &&
> +		    !reservation_object_test_signaled_rcu(resv, false))
> +			events &= ~POLLIN;
> +
> +		return events;
> +	}
> +
>  retry:
>  	seq = read_seqcount_begin(&resv->seq);
>  	rcu_read_lock();
> -- 
> 2.9.3
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch