til: introduce til_frame_plan_t and .cpu_affinity

modules/checkers w/fill_module=$module requires a consistent
mapping of cpu to fragnum since it creates a per-cpu
til_module_context_t for the fill_module.

The existing implementation for threaded rendering maximizes
performance by letting *any* scheduled to run thread advance
fragnum atomically and render the acquired fragnum
indiscriminately.  A side effect of this is any given frame, even
rendered by the same module, will have a random mapping of
cpus/threads to fragnums.

With this change, the simple til_module_t.prepare_frame() API of
returning a bare fragmenter function is changed to instead return
a "frame plan" in til_frame_plan_t.  Right now til_frame_plan_t
just contains the same fragmenter as before, but also has a
.cpu_affinity member for setting if the frame requires a stable
relationship of cpu/thread to fragnum.

Setting .cpu_affinity should be avoided if unnecessary, and that
is the default if you don't mention .cpu_affinity at all when
initializing the plan in the ergonomic manner w/designated
initializers.  This is because the way .cpu_affinity is
implemented will leave threads spinning while they poll for
*their* next fragnum using atomic intrinsics.  There's probably
some room for improvement here, but this is good enough for now
to get things working and correct.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/modules/snow/snow.c b/src/modules/snow/snow.c
index fcc40b3..a367113 100644
--- a/src/modules/snow/snow.c
+++ b/src/modules/snow/snow.c
@@ -37,9 +37,9 @@ static til_module_context_t * snow_create_context(unsigned seed, unsigned ticks,
 }
 
 
-static void snow_prepare_frame(til_module_context_t *context, unsigned ticks, til_fb_fragment_t *fragment, til_fragmenter_t *res_fragmenter)
+static void snow_prepare_frame(til_module_context_t *context, unsigned ticks, til_fb_fragment_t *fragment, til_frame_plan_t *res_frame_plan)
 {
-	*res_fragmenter = til_fragmenter_slice_per_cpu;
+	*res_frame_plan = (til_frame_plan_t){ .fragmenter = til_fragmenter_slice_per_cpu };
 }