Re: [PATCH 10/11] drm/i915/execlists: Skip direct submission if only lite-restore

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Chris Wilson (2019-03-01 10:22:30)
> Quoting Tvrtko Ursulin (2019-02-28 13:20:50)
> > 
> > On 26/02/2019 10:24, Chris Wilson wrote:
> > > If we resubmitting the active context, simply skip the submission as
> > 
> > we are
> 
> It took multiple reads to even notice that the third word was missing,
> mainly because I kept skipping the "we".
> 
> > > performing the submission from the interrupt handler has higher
> > 
> >  From the tasklet?
> 
> Yes, the interrupt handler bottom half.
> 
> > You mean wait for ctx complete and then 
> > execlists_dequeue, instead of lite-restore?
> 
> Yes. We can measure the relatively large impact of lite-restoring on
> throughput (the delay in the GPU reloading the context is quite
> noticeable), but it only affects a certain microbenchmark. In theory,
> making sure we avoid the stall while handling the interrupt and
> resubmitting should offset that cost and be much preferred for
> multi-context situations, but there the interrupt overhead on an *idle*
> system (~5-7us) is not as significant and so the impact seems not as
> significant (in fact due to quirky hw, sometimes it is preferable not to
> resubmit during an inflight CS event).

On idle systems, that lite-restore cost is visible even on top of a
normal context switch, i.e. submitting
	A, AB, [CS interrupt], B 
is marginally slower than
	A, [CS interrupt], B
!!!

/o\
Sometimes I don't get our HW at all,
-Chris
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