Move the register slow register write and readback from out of the
critical path for execlists submission and delay it until the following
worker.
Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
---
drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 48 ++++++++++++++++++---
1 file changed, 41 insertions(+), 7 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index d8b206e53660..1359314f0fd5 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -83,6 +83,9 @@ static void __intel_breadcrumbs_arm_irq(struct
intel_breadcrumbs *b)
if (!b->irq_enabled++)
irq_enable(b->irq_engine);
+
+ /* Requests may have completed before we could enable the interrupt. */
+ irq_work_queue(&b->irq_work);
}
static void __intel_breadcrumbs_disarm_irq(struct intel_breadcrumbs *b)
@@ -105,8 +108,6 @@ static void add_signaling_context(struct intel_breadcrumbs
*b,
{
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,
@@ -197,6 +198,29 @@ static void signal_irq_work(struct irq_work *work)
spin_lock(&b->irq_lock);
+ /*
+ * Keep the irq armed until the interrupt after all listeners are gone.
+ *
+ * Enabling/disabling the interrupt is rather costly, roughly a couple
+ * of hundred microseconds. If we are proactive and enable/disable
+ * the interrupt around every request that wants a breadcrumb, we
+ * quickly drown in the extra orders of magnitude of latency imposed
+ * on request submission.
+ *
+ * So we try to be lazy, and keep the interrupts enabled until no
+ * more listeners appear within a breadcrumb interrupt interval (that
+ * is until a request completes that no one cares about). The
+ * observation is that listeners come in batches, and will often
+ * listen to a bunch of requests in succession.
+ *
+ * We also try to avoid raising too many interrupts, as they may
+ * be generated by userspace batches and it is unfortunately rather
+ * too easy to drown the CPU under a flood of GPU interrupts. Thus
+ * whenever no one appears to be listening, we turn off the interrupts.
+ * Fewer interrupts should conserve power -- at the very least, fewer
+ * interrupt draw less ire from other users of the system and tools
+ * like powertop.
+ */
if (list_empty(&b->signalers))
__intel_breadcrumbs_disarm_irq(b);
@@ -251,6 +275,13 @@ static void signal_irq_work(struct irq_work *work)
i915_request_put(rq);
}
+
+ if (!READ_ONCE(b->irq_armed) && !list_empty(&b->signalers)) {
+ spin_lock(&b->irq_lock);
+ if (!list_empty(&b->signalers))
+ __intel_breadcrumbs_arm_irq(b);
+ spin_unlock(&b->irq_lock);
+ }
}
struct intel_breadcrumbs *
@@ -301,8 +332,8 @@ void intel_breadcrumbs_park(struct intel_breadcrumbs *b)
__intel_breadcrumbs_disarm_irq(b);
spin_unlock_irqrestore(&b->irq_lock, flags);
- if (!list_empty(&b->signalers))
- irq_work_queue(&b->irq_work);
+ /* Kick the work once more to drain the signalers */
+ irq_work_queue(&b->irq_work);
}
void intel_breadcrumbs_free(struct intel_breadcrumbs *b)
@@ -362,9 +393,12 @@ static void insert_breadcrumb(struct i915_request *rq,
GEM_BUG_ON(!check_signal_order(ce, rq));
set_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
- /* Check after attaching to irq, interrupt may have already fired. */
- if (__request_completed(rq))
- irq_work_queue(&b->irq_work);
+ /*
+ * Defer enabling the interrupt to after HW submission and recheck
+ * the request as it may have completed and raised the interrupt as
+ * we were attaching it into the lists.
+ */
+ irq_work_queue(&b->irq_work);
}
bool i915_request_enable_breadcrumb(struct i915_request *rq)
--
2.20.1
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