[Intel-gfx] [PATCH] drm/i915: Restore waitboost credit to the synchronous waiter

[Chris Wilson]

Ideally, we want to automagically have the GPU respond to the
instantaneous load by reclocking itself. However, reclocking occurs
relatively slowly, and to the client waiting for a result from the GPU,
too late. To compensate and reduce the client latency, we allow the
first wait from a client to boost the GPU clocks to maximum. This
overcomes the lag in autoreclocking, at the expense of forcing the GPU
clocks too high. So to offset the excessive power usage, we currently
allow a client to only boost the clocks once before we detect the GPU
is idle again. This works reasonably for say the first frame in a
benchmark, but for many more synchronous workloads (like OpenCL) we find
the GPU clocks remain too low. By noting a wait which would idle the GPU
(i.e. we just waited upon the last known request), we can give that
client the idle boost credit (for their next wait) without the 100ms
delay required for us to detect the GPU idle state. The intention is to
boost clients that are stalling in the process of feeding the GPU more
work (and who in doing so let the GPU idle), without granting boost
credits to clients that are throttling themselves (such as compositors).

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: "Zou, Nanhai" <nanhai.zou at intel.com>
Cc: Jesse Barnes <jesse.barnes at intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 92598601a232..f5aef48b93db 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1312,6 +1312,22 @@ out:
 			*timeout = 0;
 	}
 
+	if (ret == 0 && rps && req->seqno == req->ring->last_submitted_seqno) {
+		/* The GPU is now idle and this client has stalled.
+		 * Since no other client has submitted a request in the
+		 * meantime, assume that this client is the only one
+		 * supplying work to the GPU but is unable to keep that
+		 * work supplied because it is waiting. Since the GPU is
+		 * then never kept fully busy, RPS autoclocking will
+		 * keep the clocks relatively low, causing further delays.
+		 * Compensate by giving the synchronous client credit for
+		 * a waitboost next time.
+		 */
+		spin_lock(&req->i915->rps.client_lock);
+		list_del_init(&rps->link);
+		spin_unlock(&req->i915->rps.client_lock);
+	}
+
 	return ret;
 }
 
-- 
2.6.2