[PATCH 17/21] drm/i915: Convert trace-irq to the breadcrumb waiter

[Chris Wilson <chris@chris-wilson.co.uk>]

If we convert the tracing over from direct use of ring->irq_get() and
over to the breadcrumb infrastructure, we only have a single user of the
ring->irq_get and so we will be able to simplify the driver routines
(eliminating the redundant validation and irq refcounting).

v2: Move to a signaling framework based upon the waiter.
v3: Track the first-signal to avoid having to walk the rbtree everytime.
v4: Mark the signaler thread as RT priority to reduce latency in the
indirect wakeups.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_drv.h          |   8 --
 drivers/gpu/drm/i915/i915_gem.c          |   9 +-
 drivers/gpu/drm/i915/i915_trace.h        |   2 +-
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 178 +++++++++++++++++++++++++++++++
 drivers/gpu/drm/i915/intel_ringbuffer.h  |   8 +-
 5 files changed, 188 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index a71d08199d57..b0235372cfdf 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3906,14 +3906,6 @@ wait_remaining_ms_from_jiffies(unsigned long timestamp_jiffies, int to_wait_ms)
 			    schedule_timeout_uninterruptible(remaining_jiffies);
 	}
 }
-
-static inline void i915_trace_irq_get(struct intel_engine_cs *engine,
-				      struct drm_i915_gem_request *req)
-{
-	if (engine->trace_irq_req == NULL && engine->irq_get(engine))
-		i915_gem_request_assign(&engine->trace_irq_req, req);
-}
-
 static inline bool __i915_request_irq_complete(struct drm_i915_gem_request *req)
 {
 	struct intel_engine_cs *engine = req->engine;
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index fdbad07b5f42..f4e550ddaa5d 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2500,7 +2500,8 @@ i915_gem_init_seqno(struct drm_i915_private *dev_priv, u32 seqno)
 
 	/* If the seqno wraps around, we need to clear the breadcrumb rbtree */
 	if (!i915_seqno_passed(seqno, dev_priv->next_seqno)) {
-		while (intel_kick_waiters(dev_priv))
+		while (intel_kick_waiters(dev_priv) ||
+		       intel_kick_signalers(dev_priv))
 			yield();
 	}
 
@@ -2964,12 +2965,6 @@ i915_gem_retire_requests_ring(struct intel_engine_cs *engine)
 		i915_gem_object_retire__read(obj, engine->id);
 	}
 
-	if (unlikely(engine->trace_irq_req &&
-		     i915_gem_request_completed(engine->trace_irq_req))) {
-		engine->irq_put(engine);
-		i915_gem_request_assign(&engine->trace_irq_req, NULL);
-	}
-
 	WARN_ON(i915_verify_lists(engine->dev));
 }
 
diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
index 3d13fde95fdf..f59cf07184ae 100644
--- a/drivers/gpu/drm/i915/i915_trace.h
+++ b/drivers/gpu/drm/i915/i915_trace.h
@@ -490,7 +490,7 @@ TRACE_EVENT(i915_gem_ring_dispatch,
 			   __entry->ring = req->engine->id;
 			   __entry->seqno = req->seqno;
 			   __entry->flags = flags;
-			   i915_trace_irq_get(req->engine, req);
+			   intel_engine_enable_signaling(req);
 			   ),
 
 	    TP_printk("dev=%u, ring=%u, seqno=%u, flags=%x",
diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 0f5fe114c204..143891a2b68a 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -22,6 +22,8 @@
  *
  */
 
+#include <linux/kthread.h>
+
 #include "i915_drv.h"
 
 static void intel_breadcrumbs_fake_irq(unsigned long data)
@@ -321,6 +323,155 @@ out_unlock:
 	spin_unlock(&b->lock);
 }
 
+struct signal {
+	struct rb_node node;
+	struct intel_wait wait;
+	struct drm_i915_gem_request *request;
+};
+
+static bool signal_complete(struct signal *signal)
+{
+	if (signal == NULL)
+		return false;
+
+	/* If another process served as the bottom-half it may have already
+	 * signalled that this wait is already completed.
+	 */
+	if (intel_wait_complete(&signal->wait))
+		return true;
+
+	/* Carefully check if the request is complete, giving time for the
+	 * seqno to be visible or if the GPU hung.
+	 */
+	if (__i915_request_irq_complete(signal->request))
+		return true;
+
+	return false;
+}
+
+static struct signal *to_signal(struct rb_node *rb)
+{
+	return container_of(rb, struct signal, node);
+}
+
+static void signaler_set_rtpriority(void)
+{
+	 struct sched_param param = { .sched_priority = 1 };
+	 sched_setscheduler_nocheck(current, SCHED_FIFO, &param);
+}
+
+static int intel_breadcrumbs_signaler(void *arg)
+{
+	struct intel_engine_cs *engine = arg;
+	struct intel_breadcrumbs *b = &engine->breadcrumbs;
+	struct signal *signal;
+
+	/* Install ourselves with high priority to reduce signalling latency */
+	signaler_set_rtpriority();
+
+	do {
+		set_current_state(TASK_INTERRUPTIBLE);
+
+		/* We are either woken up by the interrupt bottom-half,
+		 * or by a client adding a new signaller. In both cases,
+		 * the GPU seqno may have advanced beyond our oldest signal.
+		 * If it has, propagate the signal, remove the waiter and
+		 * check again with the next oldest signal. Otherwise we
+		 * need to wait for a new interrupt from the GPU or for
+		 * a new client.
+		 */
+		signal = READ_ONCE(b->first_signal);
+		if (signal_complete(signal)) {
+			/* Wake up all other completed waiters and select the
+			 * next bottom-half for the next user interrupt.
+			 */
+			intel_engine_remove_wait(engine, &signal->wait);
+
+			i915_gem_request_unreference(signal->request);
+
+			/* Find the next oldest signal. Note that as we have
+			 * not been holding the lock, another client may
+			 * have installed an even older signal than the one
+			 * we just completed - so double check we are still
+			 * the oldest before picking the next one.
+			 */
+			spin_lock(&b->lock);
+			if (signal == b->first_signal)
+				b->first_signal = rb_next(&signal->node);
+			rb_erase(&signal->node, &b->signals);
+			spin_unlock(&b->lock);
+
+			kfree(signal);
+		} else {
+			if (kthread_should_stop())
+				break;
+
+			schedule();
+		}
+	} while (1);
+
+	return 0;
+}
+
+int intel_engine_enable_signaling(struct drm_i915_gem_request *request)
+{
+	struct intel_engine_cs *engine = request->engine;
+	struct intel_breadcrumbs *b = &engine->breadcrumbs;
+	struct rb_node *parent, **p;
+	struct signal *signal;
+	bool first, wakeup;
+
+	if (unlikely(IS_ERR(b->signaler)))
+		return PTR_ERR(b->signaler);
+
+	signal = kmalloc(sizeof(*signal), GFP_ATOMIC);
+	if (unlikely(!signal))
+		return -ENOMEM;
+
+	signal->wait.task = b->signaler;
+	signal->wait.seqno = request->seqno;
+
+	signal->request = i915_gem_request_reference(request);
+
+	/* First add ourselves into the list of waiters, but register our
+	 * bottom-half as the signaller thread. As per usual, only the oldest
+	 * waiter (not just signaller) is tasked as the bottom-half waking
+	 * up all completed waiters after the user interrupt.
+	 *
+	 * If we are the oldest waiter, enable the irq (after which we
+	 * must double check that the seqno did not complete).
+	 */
+	wakeup = intel_engine_add_wait(engine, &signal->wait);
+
+	/* Now insert ourselves into the retirement ordered list of signals
+	 * on this engine. We track the oldest seqno as that will be the
+	 * first signal to complete.
+	 */
+	spin_lock(&b->lock);
+	parent = NULL;
+	first = true;
+	p = &b->signals.rb_node;
+	while (*p) {
+		parent = *p;
+		if (i915_seqno_passed(signal->wait.seqno,
+				      to_signal(parent)->wait.seqno)) {
+			p = &parent->rb_right;
+			first = false;
+		} else
+			p = &parent->rb_left;
+	}
+	rb_link_node(&signal->node, parent, p);
+	rb_insert_color(&signal->node, &b->signals);
+	if (first)
+		smp_store_mb(b->first_signal, signal);
+	spin_unlock(&b->lock);
+
+	if (wakeup)
+		wake_up_process(b->signaler);
+
+	return 0;
+}
+
 void intel_engine_init_breadcrumbs(struct intel_engine_cs *engine)
 {
 	struct intel_breadcrumbs *b = &engine->breadcrumbs;
@@ -329,12 +480,24 @@ void intel_engine_init_breadcrumbs(struct intel_engine_cs *engine)
 	setup_timer(&b->fake_irq,
 		    intel_breadcrumbs_fake_irq,
 		    (unsigned long)engine);
+
+	/* Spawn a thread to provide a common bottom-half for all signals.
+	 * As this is an asynchronous interface we cannot steal the current
+	 * task for handling the bottom-half to the user interrupt, therefore
+	 * we create a thread to do the coherent seqno dance after the
+	 * interrupt and then signal the waitqueue (via the dma-buf/fence).
+	 */
+	b->signaler = kthread_run(intel_breadcrumbs_signaler,
+				  engine, "irq/i915:%d", engine->id);
 }
 
 void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine)
 {
 	struct intel_breadcrumbs *b = &engine->breadcrumbs;
 
+	if (!IS_ERR_OR_NULL(b->signaler))
+		kthread_stop(b->signaler);
+
 	del_timer_sync(&b->fake_irq);
 }
 
@@ -356,3 +519,18 @@ unsigned intel_kick_waiters(struct drm_i915_private *i915)
 
 	return mask;
 }
+
+unsigned intel_kick_signalers(struct drm_i915_private *i915)
+{
+	struct intel_engine_cs *engine;
+	unsigned mask = 0;
+
+	for_each_engine(engine, i915) {
+		if (unlikely(READ_ONCE(engine->breadcrumbs.first_signal))) {
+			wake_up_process(engine->breadcrumbs.signaler);
+			mask |= intel_engine_flag(engine);
+		}
+	}
+
+	return mask;
+}
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 324f85e8d540..f4bca38caef0 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -141,6 +141,8 @@ struct  i915_ctx_workarounds {
 	struct drm_i915_gem_object *obj;
 };
 
+struct drm_i915_gem_request;
+
 struct intel_engine_cs {
 	struct drm_i915_private *i915;
 	const char	*name;
@@ -179,8 +181,11 @@ struct intel_engine_cs {
 	struct intel_breadcrumbs {
 		spinlock_t lock; /* protects the lists of requests */
 		struct rb_root waiters; /* sorted by retirement, priority */
+		struct rb_root signals; /* sorted by retirement */
 		struct intel_wait *first_wait; /* oldest waiter by retirement */
 		struct task_struct *tasklet; /* bh for user interrupts */
+		struct task_struct *signaler; /* used for fence signalling */
+		void *first_signal;
 		struct timer_list fake_irq; /* used after a missed interrupt */
 		bool irq_enabled;
 		bool rpm_wakelock;
@@ -199,7 +204,6 @@ struct intel_engine_cs {
 	unsigned irq_refcount; /* protected by dev_priv->irq_lock */
 	bool		irq_posted;
 	u32		irq_enable_mask;	/* bitmask to enable ring interrupt */
-	struct drm_i915_gem_request *trace_irq_req;
 	bool __must_check (*irq_get)(struct intel_engine_cs *ring);
 	void		(*irq_put)(struct intel_engine_cs *ring);
 
@@ -540,6 +544,7 @@ bool intel_engine_add_wait(struct intel_engine_cs *engine,
 			   struct intel_wait *wait);
 void intel_engine_remove_wait(struct intel_engine_cs *engine,
 			      struct intel_wait *wait);
+int intel_engine_enable_signaling(struct drm_i915_gem_request *request);
 static inline bool intel_engine_has_waiter(struct intel_engine_cs *engine)
 {
 	return READ_ONCE(engine->breadcrumbs.tasklet);
@@ -559,5 +564,6 @@ static inline bool intel_engine_wakeup(struct intel_engine_cs *engine)
 void intel_engine_enable_fake_irq(struct intel_engine_cs *engine);
 void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine);
 unsigned intel_kick_waiters(struct drm_i915_private *i915);
+unsigned intel_kick_signalers(struct drm_i915_private *i915);
 
 #endif /* _INTEL_RINGBUFFER_H_ */
-- 
2.8.1

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