[Intel-gfx] [RFC PATCH 124/162] drm/i915/lmem: allocate HWSP in lmem

[Chris Wilson]

Quoting Matthew Auld (2020-11-30 17:17:16)
> On 27/11/2020 13:55, Chris Wilson wrote:
> > Quoting Matthew Auld (2020-11-27 12:06:40)
> >> From: Michel Thierry <michel.thierry at intel.com>
> > 
> > Rationale goes here.
> > 
> > Is this wise? HWSP is very frequently read by the CPU, and expected to
> > be cached on the CPU.
> > 
> > What do the performance profiles indicate?
> 
> Do you have a recommendation for an existing selftest or IGT to help 
> measure this?
> 
> Also are you suggesting moving this to system memory, or just using a 
> different mapping type, if it's placed in local memory? Or maybe try 
> both? Although I'm pretty sceptical about !wc for local memory.

A lot of worries go out of the window if this can be in system memory
and snooped.

For measuring, I suspect there is a lot of chaff that needs to be
removed before individual microbenchmarks like perf/request discern any
difference; although that would be a starting point. We do a lot of
completion checking during execlists interrupt processing, and there we
(cpu profiles at least) are sensitive to uncached reads.

We can trivially construct a benchmark that only shows the impact of the
WC reads; but the point where I think we would first notice from userspace
is client wakeup latency scaling: benchmarks/gem_latency, which was once
a point of major concern. Nowadays, we can couple that with a second
concern about inducing system latency from interrupt processing time.
-Chris