[Intel-gfx] [PATCH 01/18] drm/i915/execlists: Set queue priority from secondary port

[Tvrtko Ursulin]

On 09/04/2018 12:13, Chris Wilson wrote:
> We can refine our current execlists->queue_priority if we inspect
> ELSP[1] rather than the head of the unsubmitted queue. Currently, we use
> the unsubmitted queue and say that if a subsequent request is more than
> important than the current queue, we will rerun the submission tasklet

s/more than important/more important/

> to evaluate the need for preemption. However, we only want to preempt if
> we need to jump ahead of a currently executing request in ELSP. The
> second reason for running the submission tasklet is amalgamate requests
> into the active context on ELSP[0] to avoid a stall when ELSP[0] drains.
> (Though repeatedly amalgamating requests into the active context and
> triggering many lite-restore is off question gain, the goal really is to
> put a context into ELSP[1] to cover the interrupt.) So if instead of
> looking at the head of the queue, we look at the context in ELSP[1] we
> can answer both of the questions more accurately -- we don't need to
> rerun the submission tasklet unless our new request is important enough
> to feed into, at least, ELSP[1].
> 
> References: f6322eddaff7 ("drm/i915/preemption: Allow preemption between submission ports")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Michał Winiarski <michal.winiarski at intel.com>
> Cc: Michel Thierry <michel.thierry at intel.com>
> Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>   drivers/gpu/drm/i915/intel_lrc.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 3592288e4696..b47d53d284ca 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -713,7 +713,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>   			kmem_cache_free(engine->i915->priorities, p);
>   	}
>   done:
> -	execlists->queue_priority = rb ? to_priolist(rb)->priority : INT_MIN;
> +	execlists->queue_priority =
> +		port != execlists->port ? rq_prio(last) : INT_MIN;

Please bear with my questions since I am not 100% up to date with 
preemption.

Should this be rq_prio("port[1]") for future proofing? Or you really 
mean last port? In which case it wouldn't be the highest pending 
priority as kerneldoc for execlists->queue_priority says.

So this patch changes the meaning of "pending". From pending == "not 
submitted to ELSP" to pending == "not submitted to ELSP[0]". Which seems 
to make sense, although it is not the easiest job to figure out the 
consequences.

It even feels like a bugfix since it prevents tasklet scheduling when 
all ports are filled with higher priority requests than the new one.

Although I failed to understand what do we do in both cases if a new 
request arrives of higher prio than the one in ELSP[1]. Looks like 
nothing? Wait until GPU moves it to ELSP[0] and preempt then? Is this so 
because we can't safely or I misread something?

Also, don't you need to manage execlists->queue_priority after CSB 
processing now? So that it correctly reflects the priority of request in 
ELSP[1] after ELSP[0] gets completed? It seems that without it would get 
stuck at the previous value and then submission could decide to skip 
scheduling the tasklet if new priority is lower than what was in ELSP[1] 
before, and so would fail to fill ELSP[1].

>   	execlists->first = rb;
>   	if (submit)
>   		port_assign(port, last);
> 

Regards,

Tvrtko