[Intel-gfx] [PATCH v10] drm/i915: Extend LRC pinning to cover GPU context writeback

[Chris Wilson]

On Mon, Jan 18, 2016 at 05:14:26PM +0000, Tvrtko Ursulin wrote:
> 
> On 18/01/16 16:53, Chris Wilson wrote:
> >On Mon, Jan 18, 2016 at 03:02:25PM +0000, Tvrtko Ursulin wrote:
> >>-       while (!list_empty(&ring->request_list)) {
> >>-               struct drm_i915_gem_request *request;
> >>-
> >>-               request = list_first_entry(&ring->request_list,
> >>-                                          struct drm_i915_gem_request,
> >>-                                          list);
> >>-
> >>-               if (!i915_gem_request_completed(request, true))
> >>+       list_for_each_entry_safe(req, next, &ring->request_list, list) {
> >>+               if (!i915_gem_request_completed(req, true))
> >>                         break;
> >>
> >>-               i915_gem_request_retire(request);
> >>+               if (!i915.enable_execlists || !i915.enable_guc_submission) {
> >>+                       i915_gem_request_retire(req);
> >>+               } else {
> >>+                       prev_req = list_prev_entry(req, list);
> >>+                       if (prev_req)
> >>+                               i915_gem_request_retire(prev_req);
> >>+               }
> >>         }
> >>
> >>To explain, this attempts to ensure that in GuC mode requests are only
> >>unreferenced if there is a *following* *completed* request.
> >>
> >>This way, regardless of whether they are using the same or different
> >>contexts, we can be sure that the GPU has either completed the
> >>context writing, or that the unreference will not cause the final
> >>unpin of the context.
> >
> >This is the first bogus step. contexts have to be unreferenced from
> >request retire, not request free. As it stands today, this forces us to
> >hold the struct_mutex for the free (causing many foul ups along the
> >line).  The only reason why it is like that is because of execlists not
> >decoupling its context pinning inside request cancel.
> 
> What is the first bogus step? My idea of how to fix the GuC issue,
> or the mention of final unreference in relation to GPU completing
> the submission?

That we want to want to actually unreference the request. We want to
unpin the context at the appropriate juncture. At the moment, it looks
like that you are conflating those two steps: "requests are only
unreferenced". Using the retirement mechanism would mean coupling the
context unpinning into a subsequent request rather than defer retiring a
completed request, for example legacy uses active vma tracking to
accomplish the same thing. Aiui, the current claim is that we couldn't
do that since the guc may reorder contexts - except that we currently
use a global seqno so that would be bad on many levels.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre