[Intel-gfx] [PATCH v3] drm/i915/pmu: Reconstruct active state on starting busy-stats

[Tvrtko Ursulin]

On 12/01/2018 09:51, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-01-12 09:40:40)
>> So submit side doesn't work in either case, unless I am missing
>> something. Would need the pair of port manipulation and context_in to be
>> atomic.
> 
> Sure, there is a small window with the result that we either never turn
> off the stats, or turn them off one too early (and then recover if the
> my understanding of the underflow protection holds). The same problem as
> existed before the reconstruction, except now the window is much
> smaller. I'm not that scared by this (it should not explode, nor result
> in hopelessly wrong stats) so it can wait until stats enabling doesn't
> need to peek at execlists. I think you will need to postpone enabling
> until the next context-switch if we wanted to avoid the atomics; except
> that poses a problem with the igt expecting busy-start to be accurate. A
> dilemma for later.

My analysis was partially incorrect, yes, there is underflow protection 
already.

But I don't see that there is a race window where we end up with 
permanent 100% busyness before the reconstruction patch. Where do you 
see that?

The worst I see without the reconstruction is to miss the accounting of 
the batch currently in progress when stats get enabled. Which is a much 
less serious, totally ignorable event.

Regards,

Tvrtko