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

[Chris Wilson]

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
---
 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