[Bug 111090] 3% perf drop in GfxBench Manhattan 3.0, 3.1 and CarChase test-cases

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111090

--- Comment #11 from Chris Wilson <chris at chris-wilson.co.uk> ---
In a lockstats comparison run, the biggest jump in contention was for page
scanning and scheduler runqueues -- suggesting that we are causing more
mempressure and cpu switches as the result?

The most significant difference in i915.ko perf is from
 drm/i915: Move GEM object domain management from struct_mutex to local
adding the ww_acquire_context, and a extra batch of work in the many ww_mutex
we acquire for serialising the fences during submission.

I still don't think there's anything alarming in the extra kernel overhead
there, we are not letting the gpu stall while waiting for the next batch to be
submitted (at least on this baby bxt) so I think it's the mempressure/scheduler
angle.

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