[Intel-gfx] [PATCH 05/12] drm/i915/scheduler: Record all dependencies upon request construction

[Chris Wilson]

On Mon, Nov 07, 2016 at 01:30:43PM +0000, Tvrtko Ursulin wrote:
> 
> On 07/11/2016 09:30, Chris Wilson wrote:
> >On Mon, Nov 07, 2016 at 09:12:57AM +0000, Tvrtko Ursulin wrote:
> >>
> >>On 04/11/2016 15:11, Chris Wilson wrote:
> >>>On Fri, Nov 04, 2016 at 02:44:44PM +0000, Tvrtko Ursulin wrote:
> >>>>
> >>>>On 03/11/2016 11:55, Chris Wilson wrote:
> >>>>>On Thu, Nov 03, 2016 at 11:03:47AM +0000, Tvrtko Ursulin wrote:
> >>>>>>
> >>>>>>On 02/11/2016 17:50, Chris Wilson wrote:
> >>>>>>>+struct i915_dependency {
> >>>>>>>+	struct i915_priotree *signal;
> >>>>>>>+	struct list_head pre_link, post_link;
> >>>>>>>+	unsigned long flags;
> >>>>>>>+#define I915_DEPENDENCY_ALLOC BIT(0)
> >>>>>>>+};
> >>>>>>>+
> >>>>>>>+struct i915_priotree {
> >>>>>>>+	struct list_head pre_list; /* who is before us, we depend upon */
> >>>>>>>+	struct list_head post_list; /* who is after us, they depend upon us */
> >>>>>>>+};
> >>>>>>
> >>>>>>I need a picture to imagine this data structure. :(
> >>>>>
> >>>>>The names suck.
> >>>>
> >>>>When you wrote this I assumed you would respin shortly with some
> >>>>better names?
> >>>
> >>>Not yet. I kind of like
> >>>
> >>>struct i915_dependency {
> >>>	struct i915_priotree *signaler;
> >>>	struct list_head signaler_link;
> >>>	struct list_head listener_link;
> >>>};
> >>>
> >>>struct i915_priotree {
> >>>	struct list_head signalers_list; /* before us, we depend on them */
> >>>	struct list_head listeners_list; /* those after, who depend on us */
> >>>};
> >>>
> >>
> >>What is the signaler in i915_dependency?
> >
> >That would be the actual dependency. The fences have a notion of
> >waiters, but we need to track the signalers in order to perform PI.
> >Fwiw,
> >
> >+struct i915_dependency {
> >+       struct i915_priotree *signaler;
> >+       struct list_head signal_link;
> >+       struct list_head wait_link;
> >+       unsigned long flags;
> >+#define I915_DEPENDENCY_ALLOC BIT(0)
> >+};
> >+
> >+/* Requests exist in a complex web of interdependencies. Each request
> >+ * has to wait for some other request to complete before it is ready to be run
> >+ * (e.g. we have to wait until the pixels have been rendering into a texture
> >+ * before we can copy from it). We track the readiness of a request in terms
> >+ * of fences, but we also need to keep the dependency tree for the lifetime
> >+ * of the request (beyond the life of an individual fence). We use the tree
> >+ * at various points to reorder the requests whilst keeping the requests
> >+ * in order with respect to their various dependencies.
> >+ */
> >+struct i915_priotree {
> >+       struct list_head signalers_list; /* those before us, we depend upon */
> >+       struct list_head waiters_list; /* those after us, they depend upon us */
> >+};
> >
> >
> 
> req->depq is just an optimisation to avoid one allocation in the
> common case?

Because we can't handle an allocation failure at the point of use, so we
need to preallocate it at the time of request construction - so easiest
to stuff it into the request (and reasonable since we almost always need
to use - the only time we don't is on the very first request on a new
timeline).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre