[Libva] High CPU usage in __i915_wait_request when encoding video

[Steinar H. Gunderson]

[This is a resend -- I believe the first one was stuck infinitely in
 moderation, so I subscribed and re-sent.]

Hi,

I'm on a Haswell system, using the new Mesa 11 EGL/VAAPI interop to render
directly into VAAPI buffers and get H.264 video out. I have three relevant
threads:

  * Thread #1 drives OpenGL rendering into the right textures, and sends
    frame numbers (and fences) into the encoding queue.

  * Thread #2 reads from the encoding queue, waits for the fence (so that
    the GPU is done rendering) and asks VAAPI to encode the frame through
    vaBeginPicture() etc., then sends the frame numbers into the storage
    queue.

  * Thread #3 reads from the storage queue, waits for the frame to be
    done encoding (through vaSyncSurface) and stores it to disk.

Now, my problem is that thread #3 is using a lot of CPU; in particular,
__i915_wait_request uses around 30% of the total CPU of my application (which
uses almost 1.5 of the two cores when the thermal constraints come and clock
down my CPU). Looking at the stack trace from perf, this comes from the
vaSyncSurface call, which as I understand it waits for the encoder to be done
with the frame.

Is it possible to make it wait without busylooping? It seems like a strange
way to use the CPU. (I'm fine with extra latency if need be.)

/* Steinar */
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