[PATCH] drm/i915: Throttle execbuffer hogs

[Chris Wilson]

Apply backpressure to hogs that emit requests faster than the GPU can
process them by waiting for their ring to be less than half-full before
proceeding with taking the struct_mutex.

This is a gross hack to apply throttling backpressure, the long term
goal is to remove the struct_mutex contention so that each client
naturally waits, preferrably in an nonblocking fashion, for their own
resources and never blocks another within the driver at least. (Realtime
priority goals would extend to ensuring that resource contention favours
high priority clients as well.)

This only limits excessive request production and does not attempt to
throttle clients that block waiting for eviction (either global GTT or
system memory), see above for the long term goal.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 62 ++++++++++++++++++++++
 drivers/gpu/drm/i915/intel_ringbuffer.c    | 14 +----
 drivers/gpu/drm/i915/intel_ringbuffer.h    | 12 +++++
 3 files changed, 75 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 8eedf7cac493..7db857c9c443 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -753,6 +753,63 @@ static int eb_select_context(struct i915_execbuffer *eb)
 	return 0;
 }
 
+static bool __eb_ring_full(const struct intel_ring *ring)
+{
+	return READ_ONCE(ring->space) < PAGE_SIZE;
+}
+
+static struct i915_request *__eb_wait_for_ring(struct intel_ring *ring)
+{
+	struct i915_request *rq;
+
+	if (intel_ring_update_space(ring) >= PAGE_SIZE)
+		return NULL;
+
+	/*
+	 * Find a request that after waiting upon, there will be at least half
+	 * the ring available. The hystersis allows us to compete for the
+	 * shared ring and should mean that we sleep less often prior to
+	 * claiming our resources.
+	 */
+	list_for_each_entry(rq, &ring->request_list, ring_link) {
+		if (__intel_ring_space(rq->postfix,
+				       ring->emit, ring->size) > ring->size / 2)
+			break;
+	}
+	if (&rq->ring_link == &ring->request_list)
+		return NULL; /* weird, we'll check again later for real */
+
+	return i915_request_get(rq);
+}
+
+static int eb_wait_for_ring(const struct i915_execbuffer *eb)
+{
+	const struct intel_context *ce;
+	struct i915_request *rq;
+	int ret = 0;
+
+	ce = to_intel_context(eb->ctx, eb->engine);
+	if (!ce->ring) /* first use, assume empty! */
+		return 0;
+
+	if (!__eb_ring_full(ce->ring))
+		return 0;
+
+	mutex_lock(&eb->i915->drm.struct_mutex);
+	rq = __eb_wait_for_ring(ce->ring);
+	mutex_unlock(&eb->i915->drm.struct_mutex);
+	if (rq) {
+		if (i915_request_wait(rq,
+				      I915_WAIT_INTERRUPTIBLE,
+				      MAX_SCHEDULE_TIMEOUT) < 0)
+			ret = -EINTR;
+
+		i915_request_put(rq);
+	}
+
+	return ret;
+}
+
 static int eb_lookup_vmas(struct i915_execbuffer *eb)
 {
 	struct radix_tree_root *handles_vma = &eb->ctx->handles_vma;
@@ -2278,6 +2335,10 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 	if (unlikely(err))
 		goto err_destroy;
 
+	err = eb_wait_for_ring(&eb);
+	if (unlikely(err))
+		goto err_context;
+
 	/*
 	 * Take a local wakeref for preparing to dispatch the execbuf as
 	 * we expect to access the hardware fairly frequently in the
@@ -2438,6 +2499,7 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 	mutex_unlock(&dev->struct_mutex);
 err_rpm:
 	intel_runtime_pm_put(eb.i915, wakeref);
+err_context:
 	i915_gem_context_put(eb.ctx);
 err_destroy:
 	eb_destroy(&eb);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index b889b27f8aeb..15dcdbc0e77d 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -49,19 +49,6 @@ static inline u32 intel_hws_seqno_address(struct intel_engine_cs *engine)
 		I915_GEM_HWS_INDEX_ADDR);
 }
 
-static unsigned int __intel_ring_space(unsigned int head,
-				       unsigned int tail,
-				       unsigned int size)
-{
-	/*
-	 * "If the Ring Buffer Head Pointer and the Tail Pointer are on the
-	 * same cacheline, the Head Pointer must not be greater than the Tail
-	 * Pointer."
-	 */
-	GEM_BUG_ON(!is_power_of_2(size));
-	return (head - tail - CACHELINE_BYTES) & (size - 1);
-}
-
 unsigned int intel_ring_update_space(struct intel_ring *ring)
 {
 	unsigned int space;
@@ -1337,6 +1324,7 @@ intel_engine_create_ring(struct intel_engine_cs *engine,
 		return ERR_PTR(-ENOMEM);
 
 	INIT_LIST_HEAD(&ring->request_list);
+
 	ring->timeline = i915_timeline_get(timeline);
 
 	ring->size = size;
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 1398eb81dee6..4454e4c94de8 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -831,6 +831,18 @@ intel_ring_set_tail(struct intel_ring *ring, unsigned int tail)
 	return tail;
 }
 
+static inline unsigned int
+__intel_ring_space(unsigned int head, unsigned int tail, unsigned int size)
+{
+	/*
+	 * "If the Ring Buffer Head Pointer and the Tail Pointer are on the
+	 * same cacheline, the Head Pointer must not be greater than the Tail
+	 * Pointer."
+	 */
+	GEM_BUG_ON(!is_power_of_2(size));
+	return (head - tail - CACHELINE_BYTES) & (size - 1);
+}
+
 void intel_engine_write_global_seqno(struct intel_engine_cs *engine, u32 seqno);
 
 int intel_engine_setup_common(struct intel_engine_cs *engine);
-- 
2.20.1