[Intel-gfx] [PATCH 10/11] drm/i915/execlists: Skip direct submission if only lite-restore

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-02-28 13:20:50)
> 
> On 26/02/2019 10:24, Chris Wilson wrote:
> > If we resubmitting the active context, simply skip the submission as
> 
> we are

It took multiple reads to even notice that the third word was missing,
mainly because I kept skipping the "we".

> > performing the submission from the interrupt handler has higher
> 
>  From the tasklet?

Yes, the interrupt handler bottom half.

> You mean wait for ctx complete and then 
> execlists_dequeue, instead of lite-restore?

Yes. We can measure the relatively large impact of lite-restoring on
throughput (the delay in the GPU reloading the context is quite
noticeable), but it only affects a certain microbenchmark. In theory,
making sure we avoid the stall while handling the interrupt and
resubmitting should offset that cost and be much preferred for
multi-context situations, but there the interrupt overhead on an *idle*
system (~5-7us) is not as significant and so the impact seems not as
significant (in fact due to quirky hw, sometimes it is preferable not to
resubmit during an inflight CS event).

Anyway I just keep getting annoyed by the extra latency induced by
lite-restore without a good way to balance it against avoiding the CS
completion stall. And I live in fear of our tasklet being thrown to
ksoftirqd again.
-Chris