[Intel-gfx] [PATCH 4/5] drm/i915: Disable semaphore busywaits on saturated systems

[Chris Wilson]

Quoting Ville Syrjälä (2019-05-03 15:04:57)
> On Mon, Apr 29, 2019 at 07:00:19PM +0100, Chris Wilson wrote:
> > Asking the GPU to busywait on a memory address, perhaps not unexpectedly
> > in hindsight for a shared system, leads to bus contention that affects
> > CPU programs trying to concurrently access memory. This can manifest as
> > a drop in transcode throughput on highly over-saturated workloads.
> 
> We can't use the singalling semaphore variant?

That requires us to broadcast a signal to each engine (for which I can
hear the cries of cross-powerwell wakes), and currently does not work
with execlists + context-id==0. Or at least it failed in my testing.
-Chris