[Intel-gfx] [PATCH 07/31] drm/i915/execlists: Direct submission of new requests (avoid tasklet/ksoftirqd)

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-06-28 13:29:32)
> 
> On 28/06/2018 13:11, Chris Wilson wrote:
> > Quoting Chris Wilson (2018-06-28 13:07:51)
> >> Quoting Tvrtko Ursulin (2018-06-28 12:56:56)
> >>> And tasklet kick from intel_enable_engine_stats, hm yep. But wouldn't
> >>> taking the timeline lock around active state reconstruction solve that
> >>> simpler?
> >>
> >> Can you? We probably can. (That one was a very recent discovery and
> >> quick fix.)
> > 
> > The biggest issue being whether or not the same locking applies equally
> > to all submission backends. That's not yet true, but then again we don't
> > use stats everywhere. So whether or not that's an issue, I don't know,
> > but it's enough to make me want to punt changing the locking inside
> > intel_enable_engine_stats to a separate patch.
> 
> Big benefit is removing the extra tasklet schedule from engine stats 
> which is in fact even racy.

It's racy, but the tasklet being run more often than required is just
wasted effort. Unless you think we can get ourselves into a loop here?

> We need the state reconstruction to be 
> atomic so I think it really needs to be under the engine lock.
> 
> tasklet_disable/enable can then also be dropped I think.
> 
> To which patch in this series that belongs is the question. Last one I 
> think, when all is in place that port updates are protected by the 
> timeline lock.

I definitely support it being a new patch. I don't think the race is a
problem that requires preventative work.
-Chris