[Intel-gfx] [PATCH] drm/i915/gt: Schedule request retirement when submission idles

[Chris Wilson]

The major drawback of commit 7e34f4e4aad3 ("drm/i915/gen8+: Add RC6 CTX
corruption WA") is that it disables RC6 while Skylake (and friends) is
active, and we do not consider the GPU idle until all outstanding
requests have been retired and the engine switched over to the kernel
context. If userspace is idle, this task falls onto our background idle
worker, which only runs roughly once a second, meaning that userspace has
to have been idle for a couple of seconds before we enable RC6 again.
Naturally, this causes us to consume considerably more energy than
before as powersaving is effectively disabled while a display server
(here's looking at you Xorg) is running.

As execlists will get a completion event as the last context is
completed and the GPU goes idle, we can use our submission tasklet to
notice when the GPU is idle and kick the retire worker. Thus during
light workloads, we will do much more work to idle the GPU faster...
Hopefully with commensurate power saving!

References: https://bugs.freedesktop.org/show_bug.cgi?id=112315
References: 7e34f4e4aad3 ("drm/i915/gen8+: Add RC6 CTX corruption WA")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_gt_requests.h |  9 ++++++++-
 drivers/gpu/drm/i915/gt/intel_lrc.c         | 11 +++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gt_requests.h b/drivers/gpu/drm/i915/gt/intel_gt_requests.h
index fde546424c63..94b8758a45d9 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_requests.h
+++ b/drivers/gpu/drm/i915/gt/intel_gt_requests.h
@@ -7,7 +7,9 @@
 #ifndef INTEL_GT_REQUESTS_H
 #define INTEL_GT_REQUESTS_H
 
-struct intel_gt;
+#include <linux/workqueue.h>
+
+#include "intel_gt_types.h"
 
 long intel_gt_retire_requests_timeout(struct intel_gt *gt, long timeout);
 static inline void intel_gt_retire_requests(struct intel_gt *gt)
@@ -15,6 +17,11 @@ static inline void intel_gt_retire_requests(struct intel_gt *gt)
 	intel_gt_retire_requests_timeout(gt, 0);
 }
 
+static inline void intel_gt_schedule_retire_requests(struct intel_gt *gt)
+{
+	mod_delayed_work(system_wq, &gt->requests.retire_work, 0);
+}
+
 int intel_gt_wait_for_idle(struct intel_gt *gt, long timeout);
 
 void intel_gt_init_requests(struct intel_gt *gt);
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 33ce258d484f..4485fe3e5066 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -142,6 +142,7 @@
 #include "intel_engine_pm.h"
 #include "intel_gt.h"
 #include "intel_gt_pm.h"
+#include "intel_gt_requests.h"
 #include "intel_lrc_reg.h"
 #include "intel_mocs.h"
 #include "intel_reset.h"
@@ -2278,6 +2279,16 @@ static void execlists_submission_tasklet(unsigned long data)
 		if (timeout && preempt_timeout(engine))
 			preempt_reset(engine);
 	}
+
+	/*
+	 * If the GPU is currently idle, retire the outstanding completed
+	 * requests. This will allow us to enter soft-rc6 as soon as possible,
+	 * albeit at the cost of running the retire worker much more frequently
+	 * (over the entire GT not just this engine) and emitting more idle
+	 * barriers (i.e. kernel context switches) which may some extra latency.
+	 */
+	if (!execlists_active(&engine->execlists))
+		intel_gt_schedule_retire_requests(engine->gt);
 }
 
 static void __execlists_kick(struct intel_engine_execlists *execlists)
-- 
2.24.0