[Bug 108598] [GEN9] 20% perf drop in windowed/composited GpuTest Triangle

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108598

--- Comment #9 from Chris Wilson <chris at chris-wilson.co.uk> ---
Had a stable 2% drop on bxt. The effect was due to reordering the requests to
make i915_spin_request() more likely to be taken. The cost was not from
reordering the requests themselves, but the act of busywaiting.

i.e.

diff --git a/drivers/gpu/drm/i915/i915_request.c
b/drivers/gpu/drm/i915/i915_request.c
index abd4dacbab8e..f5d4659a4aa0 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1230,6 +1230,11 @@ long i915_request_wait(struct i915_request *rq,
        if (!timeout)
                return -ETIME;

+       /* Optimistic short spin before touching IRQs */
+       wait.seqno = i915_request_global_seqno(rq);
+       if (wait.seqno && __i915_spin_request(rq, wait.seqno, state, 5))
+               return timeout;
+
        trace_i915_request_wait_begin(rq, flags);

        add_wait_queue(&rq->execute, &exec);
@@ -1266,10 +1271,6 @@ long i915_request_wait(struct i915_request *rq,
        GEM_BUG_ON(!intel_wait_has_seqno(&wait));
        GEM_BUG_ON(!i915_sw_fence_signaled(&rq->submit));

-       /* Optimistic short spin before touching IRQs */
-       if (__i915_spin_request(rq, wait.seqno, state, 5))
-               goto complete;
-

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