[Intel-gfx] [PATCH 10/12] drm/i915/execlists: Reduce lock contention between schedule/submit_request

[Tvrtko Ursulin]

On 15/05/2017 11:55, Chris Wilson wrote:
> On Mon, May 15, 2017 at 11:51:52AM +0100, Tvrtko Ursulin wrote:
>>
>> On 11/05/2017 20:59, Chris Wilson wrote:
>>> If we do not require to perform priority bumping, and we haven't yet
>>> submitted the request, we can update its priority in situ and skip
>>> acquiring the engine locks -- thus avoiding any contention between us
>>> and submit/execute.
>>>
>>> v2: Remove the stack element from the list if we can do the early
>>> assignment.
>>>
>>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>>> ---
>>> drivers/gpu/drm/i915/intel_lrc.c | 13 +++++++++++++
>>> 1 file changed, 13 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
>>> index 7e41529bd074..3cfb4b145b75 100644
>>> --- a/drivers/gpu/drm/i915/intel_lrc.c
>>> +++ b/drivers/gpu/drm/i915/intel_lrc.c
>>> @@ -767,6 +767,19 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
>>> 		list_safe_reset_next(dep, p, dfs_link);
>>> 	}
>>>
>>> +	/* If we didn't need to bump any existing priorities, and we haven't
>>> +	 * yet submitted this request (i.e. there is no potential race with
>>> +	 * execlists_submit_request()), we can set our own priority and skip
>>> +	 * acquiring the engine locks.
>>> +	 */
>>> +	if (request->priotree.priority == INT_MIN) {
>>> +		GEM_BUG_ON(!list_empty(&request->priotree.link));
>>> +		request->priotree.priority = prio;
>>> +		if (stack.dfs_link.next == stack.dfs_link.prev)
>>> +			return;
>>> +		__list_del_entry(&stack.dfs_link);
>>
>> Hm is early processing the out of queue request with no dependencies
>> safe? Recently you fixed a race in this area, which AFAIK was about
>> doing that outside the engine lock and then request becoming ready
>> in parallel, racing with the FIFO order.
>
> Yes. In this case we know that we have commited the request as we are in
> i915_add_request (the only time we may schedule with prio == INT_MIN).

Okay, sounds safe in that case.

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

Regards,

Tvrtko