[Intel-gfx] [PATCH] RFC drm/i915: Slaughter the thundering i915_wait_request herd

[Gong, Zhipeng]

> From: Chris Wilson [mailto:chris at chris-wilson.co.uk]
> On Tue, Nov 03, 2015 at 01:31:22PM +0000, Gong, Zhipeng wrote:
> >
> > > From: Chris Wilson [mailto:chris at chris-wilson.co.uk]
> > >
> > > Do you also have a relative perf statistics like op/s we can compare
> > > to make sure we aren't just stalling the whole system?
> > >
> > Could you please provide the commands about how to check it?
> 
> I was presuming your workload has some measure of efficiency/throughput?
> It is one thing to say we are using 10% less CPU (per second), but the task is
> running 2x as long!
We use execute time as a measurement, the patch affects the execution time 
for our cases slightly.

Exec time(s)    |   w/o patch   |   w/patch
-----------------------------------------------
BDW async 1     |    65.00      |    65.25
BDW async 5     |    68.50      |    66.42

> 
> > > How much cpu time is left in the i915_wait_request branch? i.e. how
> > > close to the limit are we with chasing this path?
> > Could you please provide the commands here either? :)
> 
> Check the perf callgraph.

Now the most of time is in io_schedule_timeout
__i915_wait_request  
|--64.04%-- io_schedule_timeout
|--22.04%-- intel_engine_add_wakeup
|--3.13%-- prepare_to_wait
|--2.99%-- gen6_rps_boost
|-...