Re: [PATCH] drm/i915/execlists: Tweak virtual unsubmission

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

On 14/10/2019 10:59, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-10-14 10:50:25)
>>
>> On 14/10/2019 10:41, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2019-10-14 10:34:31)
>>>>
>>>> On 13/10/2019 21:30, Chris Wilson wrote:
>>>>> Since commit e2144503bf3b ("drm/i915: Prevent bonded requests from
>>>>> overtaking each other on preemption") we have restricted requests to run
>>>>> on their chosen engine across preemption events. We can take this
>>>>> restriction into account to know that we will want to resubmit those
>>>>> requests onto the same physical engine, and so can shortcircuit the
>>>>> virtual engine selection process and keep the request on the same
>>>>> engine during unwind.
>>>>>
>>>>> References: e2144503bf3b ("drm/i915: Prevent bonded requests from overtaking each other on preemption")
>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>> ---
>>>>>     drivers/gpu/drm/i915/gt/intel_lrc.c | 6 +++---
>>>>>     drivers/gpu/drm/i915/i915_request.c | 2 +-
>>>>>     2 files changed, 4 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> index e6bf633b48d5..03732e3f5ec7 100644
>>>>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> @@ -895,7 +895,6 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
>>>>>         list_for_each_entry_safe_reverse(rq, rn,
>>>>>                                          &engine->active.requests,
>>>>>                                          sched.link) {
>>>>> -             struct intel_engine_cs *owner;
>>>>>     
>>>>>                 if (i915_request_completed(rq))
>>>>>                         continue; /* XXX */
>>>>> @@ -910,8 +909,7 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
>>>>>                  * engine so that it can be moved across onto another physical
>>>>>                  * engine as load dictates.
>>>>>                  */
>>>>> -             owner = rq->hw_context->engine;
>>>>> -             if (likely(owner == engine)) {
>>>>> +             if (likely(rq->execution_mask == engine->mask)) {
>>>>>                         GEM_BUG_ON(rq_prio(rq) == I915_PRIORITY_INVALID);
>>>>>                         if (rq_prio(rq) != prio) {
>>>>>                                 prio = rq_prio(rq);
>>>>> @@ -922,6 +920,8 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
>>>>>                         list_move(&rq->sched.link, pl);
>>>>>                         active = rq;
>>>>>                 } else {
>>>>> +                     struct intel_engine_cs *owner = rq->hw_context->engine;
>>>>
>>>> I guess there is some benefit in doing fewer operations as long as we
>>>> are fixing the engine anyway (at the moment at least).
>>>>
>>>> However on this branch here the concern was request completion racing
>>>> with preemption handling and with this change the breadcrumb will not
>>>> get canceled any longer and may get signaled on the virtual engine.
>>>> Which then leads to the explosion this branch fixed. At least that's
>>>> what I remembered in combination with the comment below..
>>>
>>> No, we don't change back to the virtual engine, so that is not an issue.
>>> The problem was only because of the rq->engine = owner where the
>>> breadcrumbs were still on the previous engine lists and assumed to be
>>> under that engine->breadcrumbs.lock (but would in future be assumed to be
>>> under rq->engine->breadcrumbs.lock).
>>
>> Breadcrumb signaling can only be set up on the physical engine? Hm, must
>> be fine since without preemption that would be the scenario exactly.
>> Okay, I see there is r-b from Ram already so no need for another one.
> 
> With no disrespect to Ram, as the expert you raised a technical point that
> I would be happier to record as resolved with an r-b from yourself.

I went back to the patch I reviewed in July and it checks out.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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