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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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?

Also I don't understand how would you decouple context and request lifetime?

Maybe we can ignore execlist mode for the moment and just consider the 
GuC which, as much as I understand it, has a simpler and fully aligned 
request/context/lrc lifetime of:

* reference and pin and request creation
* unpin and unreference at retire

Where retire is decoupled from actual GPU activity, or maybe better say 
indirectly driven.

Execlists bolt on a parallel another instance reference and pin on top, 
with different lifetime rules so maybe ignore that for the GuC 
discussion. Just to figure out what you have in mind.

Regards,

Tvrtko
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