[Intel-gfx] [PATCH v3] drm/i915: Use exponential backoff for wait_for()

[Chris Wilson]

Quoting Michał Winiarski (2017-11-24 12:37:56)
> Since we see the effects for GuC preeption, let's gather some evidence.
> 
> (SKL)
> intel_guc_send_mmio latency: 100 rounds of gem_exec_latency --r '*-preemption'
> 
> drm-tip:
>      usecs               : count     distribution
>          0 -> 1          : 0        |                                        |
>          2 -> 3          : 0        |                                        |
>          4 -> 7          : 0        |                                        |
>          8 -> 15         : 44       |                                        |
>         16 -> 31         : 1088     |                                        |
>         32 -> 63         : 832      |                                        |
>         64 -> 127        : 0        |                                        |
>        128 -> 255        : 0        |                                        |
>        256 -> 511        : 12       |                                        |
>        512 -> 1023       : 0        |                                        |
>       1024 -> 2047       : 29899    |*********                               |
>       2048 -> 4095       : 131033   |****************************************|

Such pretty graphs. Reminds me of the bpf hist output, I wonder if we
could create a tracepoint/kprobe that would output a histogram for each
waiter (filterable ofc). Benefit? Just thinking of tuning the
spin/sleep, in which case overall metrics are best
(intel_eait_for_register needs to be optimised for the typical case). I
am wondering if we could tune the spin period down to 5us, 2us? And then
have the 10us sleep.

We would also need a typical workload to run, it's profile-guided
optimisation after all. Hmm.
-Chris