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

[Gong, Zhipeng]

> -----Original Message-----
> From: Chris Wilson [mailto:chris at chris-wilson.co.uk]
> Sent: Wednesday, November 04, 2015 5:54 PM
> On Wed, Nov 04, 2015 at 06:19:33AM +0000, Gong, Zhipeng wrote:
> > > 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
> 
> That's reassuring.
> 
> > >
> > > > > 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
> > |-...
> 
> No more busywaits, and most of the time is spent kicking the next process or
> doing the insertion sort into the waiting rbtree.
> 
> What's the ratio now of __i915_wait_request to the next hot function?
> And who are the chief callers of __i915_wait_request?
> -Chris
Please check the attachments for the details, I post a piece of it here:
|--17.89%-- i915_gem_object_sync
         |--73.19%-- __i915_wait_request
         |--12.60%-- i915_gem_object_retire_request
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