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

[Chris Wilson]

On Sun, Aug 28, 2016 at 09:33:54PM +0100, Chris Wilson wrote:
> 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
> 
> Hmm, this only really applies to the idle case.
> reservation_object_test_signaled_rcu() is still a major bottleneck when
> busy, due to the dance inside reservation_object_test_signaled_single()

The fix is not difficult, just requires extending the seqlock to catch
the RCU race (i.e. earlier patches). I'll resend that series in the
morning.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre