drm/i915: Signal first fence from irq handler if complete

As execlists and other non-semaphore multi-engine devices coordinate
between engines using interrupts, we can shave off a few 10s of
microsecond of scheduling latency by doing the fence signaling from the
interrupt as opposed to a RT kthread. (Realistically the delay adds
about 1% to an individual cross-engine workload.) We only signal the
first fence in order to limit the amount of work we move into the
interrupt handler. We also have to remember that our breadcrumbs may be
unordered with respect to the interrupt and so we still require the
waiter process to perform some heavyweight coherency fixups, as well as
traversing the tree of waiters.

v2: No need for early exit in irq handler - it breaks the flow between
patches and prevents the tracepoint
v3: Restore rcu hold across irq signaling of request

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227205850.2828-2-chris@chris-wilson.co.uk

1 files changed, 6 insertions, 7 deletions

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 933874ddea75..71152656ebbf 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -4060,9 +4060,9 @@ __i915_request_irq_complete(const struct drm_i915_gem_request *req)
 	 * is woken.
 	 */
 	if (engine->irq_seqno_barrier &&
-	    rcu_access_pointer(engine->breadcrumbs.irq_seqno_bh) == current &&
 	    test_and_clear_bit(ENGINE_IRQ_BREADCRUMB, &engine->irq_posted)) {
-		struct task_struct *tsk;
+		struct intel_breadcrumbs *b = &engine->breadcrumbs;
+		unsigned long flags;
 
 		/* The ordering of irq_posted versus applying the barrier
 		 * is crucial. The clearing of the current irq_posted must
@@ -4084,17 +4084,16 @@ __i915_request_irq_complete(const struct drm_i915_gem_request *req)
 		 * the seqno before we believe it coherent since they see
 		 * irq_posted == false but we are still running).
 		 */
-		rcu_read_lock();
-		tsk = rcu_dereference(engine->breadcrumbs.irq_seqno_bh);
-		if (tsk && tsk != current)
+		spin_lock_irqsave(&b->lock, flags);
+		if (b->first_wait && b->first_wait->tsk != current)
 			/* Note that if the bottom-half is changed as we
 			 * are sending the wake-up, the new bottom-half will
 			 * be woken by whomever made the change. We only have
 			 * to worry about when we steal the irq-posted for
 			 * ourself.
 			 */
-			wake_up_process(tsk);
-		rcu_read_unlock();
+			wake_up_process(b->first_wait->tsk);
+		spin_unlock_irqrestore(&b->lock, flags);
 
 		if (__i915_gem_request_completed(req, seqno))
 			return true;