Re: [Intel-gfx] [PATCH 06/16] drm/i915/gt: Decouple completed requests on unwind

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

On 25/11/2020 10:21, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2020-11-25 09:15:25)
>>
>> On 24/11/2020 17:31, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2020-11-24 17:13:02)
>>>>
>>>> On 24/11/2020 11:42, Chris Wilson wrote:
>>>>> Since the introduction of preempt-to-busy, requests can complete in the
>>>>> background, even while they are not on the engine->active.requests list.
>>>>> As such, the engine->active.request list itself is not in strict
>>>>> retirement order, and we have to scan the entire list while unwinding to
>>>>> not miss any. However, if the request is completed we currently leave it
>>>>> on the list [until retirement], but we could just as simply remove it
>>>>> and stop treating it as active. We would only have to then traverse it
>>>>> once while unwinding in quick succession.
>>>>>
>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>> ---
>>>>>     drivers/gpu/drm/i915/gt/intel_lrc.c | 6 ++++--
>>>>>     drivers/gpu/drm/i915/i915_request.c | 3 ++-
>>>>>     2 files changed, 6 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> index 30aa59fb7271..cf11cbac241b 100644
>>>>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> @@ -1116,8 +1116,10 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
>>>>>         list_for_each_entry_safe_reverse(rq, rn,
>>>>>                                          &engine->active.requests,
>>>>>                                          sched.link) {
>>>>> -             if (i915_request_completed(rq))
>>>>> -                     continue; /* XXX */
>>>>> +             if (i915_request_completed(rq)) {
>>>>> +                     list_del_init(&rq->sched.link);
>>>>> +                     continue;
>>>>> +             }
>>>>>     
>>>>>                 __i915_request_unsubmit(rq);
>>>>>     
>>>>> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
>>>>> index 8d7d29c9e375..a9db1376b996 100644
>>>>> --- a/drivers/gpu/drm/i915/i915_request.c
>>>>> +++ b/drivers/gpu/drm/i915/i915_request.c
>>>>> @@ -321,7 +321,8 @@ bool i915_request_retire(struct i915_request *rq)
>>>>>          * after removing the breadcrumb and signaling it, so that we do not
>>>>>          * inadvertently attach the breadcrumb to a completed request.
>>>>>          */
>>>>> -     remove_from_engine(rq);
>>>>> +     if (!list_empty(&rq->sched.link))
>>>>> +             remove_from_engine(rq);
>>>>
>>>> The list_empty check is unlocked so is list_del_init in
>>>> remove_from_engine safe on potentially already unlinked request or it
>>>> needs to re-check under the lock?
>>>
>>> It's safe. The unwind is under the lock, and remove_from_engine takes
>>> the lock, and both do list_del_init() which is a no-op if already
>>> removed. And the end state of the flag bits is the same on each path. We
>>> can skip the __notify_execute_cb_imm() since we know in unwind it is
>>> executing and there should be no cb.
>>>
>>> The test before we take the lock is only allowed to skip the active.lock
>>> if it sees the list is already decoupled, in which case we can leave it
>>> to the unwind to remove it from the engine (and we know that the request
>>> can only have been inflight prior to completion). Since the test is not
>>> locked, we don't serialise with the removal, but the list_del_init is
>>> the last action on the request so there is no window where the unwind is
>>> accessing the request after it may have been retired.
>>>
>>> list_move() will not confuse list_empty(), as although it does a
>>> list_del_entry, it is not temporarily re-initialised to an empty list.
>>
>> List_del_init is indeed safe. List_move.. which one you think can race
>> with retire? Preempt-to-busy unwinding an almost completed request yet
>> again? Or even preempt timeout racing with completion?
> 
> Here in unwind. We pass the completion check, but the request may still
> be running and complete at any time (until we submit & ack the new ELSP).
> So an unlocked list_empty check during retire can race with any of the
> list_move during unwind and resubmit. (On resubmit, we check completed
> under the lock and drop the request in __i915_request_submit which
> should also leave it in a consistent state as if we had called
> remove_from_engine.)

Right, yes, that seems safe as well. Only new problem could have been a 
false negative, meaning remote_from_engine _not_ scheduled by mistake if 
a transient false list_empty condition.

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

Regards,

Tvrtko


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