Re: [PATCH 07/10] drm/i915/gt: Hold context/request reference while breadcrumbs are active

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On 20/07/2020 10:23, Chris Wilson wrote:
Currently we hold no actual reference to the request nor context while
they are attached to a breadcrumb. To avoid freeing the request/context
too early, we serialise with cancel-breadcrumbs by taking the irq
spinlock in i915_request_retire(). The alternative is to take a
reference for a new breadcrumb and release it upon signaling; removing
the more frequently hit contention point in i915_request_retire().

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 58 ++++++++++++++++-----
  drivers/gpu/drm/i915/i915_request.c         |  9 ++--
  2 files changed, 48 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index d6008034869f..59e8cd505569 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -29,6 +29,7 @@
  #include "i915_drv.h"
  #include "i915_trace.h"
  #include "intel_breadcrumbs.h"
+#include "intel_context.h"
  #include "intel_gt_pm.h"
  #include "intel_gt_requests.h"
@@ -100,6 +101,22 @@ static void __intel_breadcrumbs_disarm_irq(struct intel_breadcrumbs *b)
  	intel_gt_pm_put_async(b->irq_engine->gt);
  }
+static void add_signaling_context(struct intel_breadcrumbs *b,
+				  struct intel_context *ce)
+{
+	intel_context_get(ce);
+	list_add_tail(&ce->signal_link, &b->signalers);
+	if (list_is_first(&ce->signal_link, &b->signalers))
+		__intel_breadcrumbs_arm_irq(b);
+}
+
+static void remove_signaling_context(struct intel_breadcrumbs *b,
+				     struct intel_context *ce)
+{
+	list_del(&ce->signal_link);
+	intel_context_put(ce);
+}
+
  static inline bool __request_completed(const struct i915_request *rq)
  {
  	return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
@@ -108,6 +125,9 @@ static inline bool __request_completed(const struct i915_request *rq)
  __maybe_unused static bool
  check_signal_order(struct intel_context *ce, struct i915_request *rq)
  {
+	if (rq->context != ce)
+		return false;
+
  	if (!list_is_last(&rq->signal_link, &ce->signals) &&
  	    i915_seqno_passed(rq->fence.seqno,
  			      list_next_entry(rq, signal_link)->fence.seqno))
@@ -162,7 +182,6 @@ static bool __signal_request(struct i915_request *rq, struct list_head *signals)
  	if (!__dma_fence_signal(&rq->fence))
  		return false;
- i915_request_get(rq);
  	list_add_tail(&rq->signal_link, signals);
  	return true;
  }
@@ -198,7 +217,8 @@ static void signal_irq_work(struct irq_work *work)
  			 * spinlock as the callback chain may end up adding
  			 * more signalers to the same context or engine.
  			 */
-			__signal_request(rq, &signal);
+			if (!__signal_request(rq, &signal))
+				i915_request_put(rq);

Looks like __signal_request could do the request put but doesn't matter hugely.

  		}
/*
@@ -210,7 +230,7 @@ static void signal_irq_work(struct irq_work *work)
  			/* Advance the list to the first incomplete request */
  			__list_del_many(&ce->signals, pos);
  			if (&ce->signals == pos) { /* now empty */
-				list_del_init(&ce->signal_link);
+				remove_signaling_context(b, ce);
  				add_retire(b, ce->timeline);
  			}
  		}
@@ -282,6 +302,8 @@ void intel_breadcrumbs_park(struct intel_breadcrumbs *b)
  	spin_lock_irqsave(&b->irq_lock, flags);
  	if (b->irq_armed)
  		__intel_breadcrumbs_disarm_irq(b);
+	if (!list_empty(&b->signalers))
+		irq_work_queue(&b->irq_work);
  	spin_unlock_irqrestore(&b->irq_lock, flags);
  }
@@ -299,6 +321,8 @@ static void insert_breadcrumb(struct i915_request *rq,
  	if (test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
  		return;
+ i915_request_get(rq);
+
  	/*
  	 * If the request is already completed, we can transfer it
  	 * straight onto a signaled list, and queue the irq worker for
@@ -307,11 +331,11 @@ static void insert_breadcrumb(struct i915_request *rq,
  	if (__request_completed(rq)) {
  		if (__signal_request(rq, &b->signaled_requests))
  			irq_work_queue(&b->irq_work);
+		else
+			i915_request_put(rq);
  		return;
  	}
- __intel_breadcrumbs_arm_irq(b);
-
  	/*
  	 * We keep the seqno in retirement order, so we can break
  	 * inside intel_engine_signal_breadcrumbs as soon as we've
@@ -326,16 +350,20 @@ static void insert_breadcrumb(struct i915_request *rq,
  	 * start looking for our insertion point from the tail of
  	 * the list.
  	 */
-	list_for_each_prev(pos, &ce->signals) {
-		struct i915_request *it =
-			list_entry(pos, typeof(*it), signal_link);
-
-		if (i915_seqno_passed(rq->fence.seqno, it->fence.seqno))
-			break;
+	if (list_empty(&ce->signals)) {
+		add_signaling_context(b, ce);
+		GEM_BUG_ON(!b->irq_armed);
+		pos = &ce->signals;
+	} else {
+		list_for_each_prev(pos, &ce->signals) {
+			struct i915_request *it =
+				list_entry(pos, typeof(*it), signal_link);
+
+			if (i915_seqno_passed(rq->fence.seqno, it->fence.seqno))
+				break;
+		}
  	}
  	list_add(&rq->signal_link, pos);
-	if (pos == &ce->signals) /* catch transitions from empty list */
-		list_move_tail(&ce->signal_link, &b->signalers);
  	GEM_BUG_ON(!check_signal_order(ce, rq));
  	set_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
@@ -416,9 +444,10 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq) list_del(&rq->signal_link);
  		if (list_empty(&ce->signals))
-			list_del_init(&ce->signal_link);
+			remove_signaling_context(b, ce);
clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
+		i915_request_put(rq);
  	}
  	spin_unlock(&b->irq_lock);
  }
@@ -433,6 +462,7 @@ void intel_engine_print_breadcrumbs(struct intel_engine_cs *engine,
  	if (!b || list_empty(&b->signalers))
  		return;
+ drm_printf(p, "IRQ: %s\n", enableddisabled(b->irq_armed));
  	drm_printf(p, "Signals:\n");
spin_lock_irq(&b->irq_lock);
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 394587e70a2d..4ebb0f144ac4 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -300,12 +300,11 @@ bool i915_request_retire(struct i915_request *rq)
  		__i915_request_fill(rq, POISON_FREE);
  	rq->ring->head = rq->postfix;
- spin_lock_irq(&rq->lock);
-	if (!i915_request_signaled(rq))
+	if (!i915_request_signaled(rq)) {
+		spin_lock_irq(&rq->lock);
  		dma_fence_signal_locked(&rq->fence);
-	if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &rq->fence.flags))
-		i915_request_cancel_breadcrumb(rq);
-	spin_unlock_irq(&rq->lock);
+		spin_unlock_irq(&rq->lock);
+	}
if (i915_request_has_waitboost(rq)) {
  		GEM_BUG_ON(!atomic_read(&rq->engine->gt->rps.num_waiters));


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Regards,

Tvrtko
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