r600g: Trying to remove the busywait for fence completion, but hitting inexplicable behaviour

[Alan Swanson]

On Tue, 2012-01-31 at 13:16 +0000, Simon Farnsworth wrote:
> Hello,
> 
> When profiling my workload on an AMD E-350 (PALM GPU) to see why it still
> wasn't performing well with Jerome's WIP macrotiling patches, I noticed that
> r600_fence_finish was taking 10% of my CPU time. I determined experimentally
> that changing from sched_yield() to os_time_sleep(10) fixed this and
> resolved my last performance issue on AMD Fusion as compared to Intel Atom,
> but felt that this was hacky.

No, you were right the first time, sched_yield should definitely not be
being used in this busy wait under Linux (even with its preceding few
spins).

Back in 2003 when Linux 2.5 changed sched_yield to move processes from
run queue to the expired queue instead, its use was discussed on the DRI
devel list.

http://lwn.net/Articles/31462/
http://thread.gmane.org/gmane.comp.video.dri.devel/5455/focus=6779

There stills seems to be five uses that should be checked.

src/gallium/drivers/r600/r600_pipe.c
src/gallium/drivers/nouveau/nouveau_fence.c
src/gallium/auxiliary/pipebuffer/pb_buffer_fenced.c
src/gallium/winsys/radeon/drm/radeon_drm_bo.c (*2)

-- 
Alan.