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

[Tvrtko Ursulin]

On 28/06/2018 13:35, Chris Wilson wrote:
> 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.

I think it has to be part of this series as the last, or before it, 
since otherwise the perf_pmu test could/would/should start failing.

Because it is this series which breaks the assumption in 
intel_enable_engine_stats that ports can be sampled safely while the 
tasklet is disabled.

I don't think the extra tasklet schedule fixes it - it cannot help 
correcting the state once the above races with direct submission.

So that extra/new tasklet schedule also goes away.

Regards,

Tvrtko