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

[Tvrtko Ursulin]

On 18/01/16 20:47, Chris Wilson wrote:
> 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

What would be the appropriate juncture? With GuC we don't have the 
equivalent of context complete irq.

> 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.

I don't know legacy. :( I can see that request/context lifetime is 
coupled there and associated with request creation to retirement.

Does it have the same problem of seqno signaling completion before the 
GPU is done with writing out the context image and how does it solve that?

Regards,

Tvrtko