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<b><a class="bz_bug_link
bz_status_NEW "
title="NEW - [GEN9] 20% perf drop in windowed/composited GpuTest Triangle"
href="https://bugs.freedesktop.org/show_bug.cgi?id=108598#c9">Comment # 9</a>
on <a class="bz_bug_link
bz_status_NEW "
title="NEW - [GEN9] 20% perf drop in windowed/composited GpuTest Triangle"
href="https://bugs.freedesktop.org/show_bug.cgi?id=108598">bug 108598</a>
from <span class="vcard"><a class="email" href="mailto:chris@chris-wilson.co.uk" title="Chris Wilson <chris@chris-wilson.co.uk>"> <span class="fn">Chris Wilson</span></a>
</span></b>
<pre>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;
-</pre>
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