drm/amdkfd: use a high priority workqueue for IH work

In systems under heavy load the IH work may experience significant
scheduling delays.

Under load + system workqueue:
Max Latency: 7.023695 ms
Avg Latency: 0.263994 ms

Under load + high priority workqueue:
Max Latency: 1.162568 ms
Avg Latency: 0.163213 ms

Further work is required to measure the impact of per-cpu settings on IH
performance.

Signed-off-by: Andres Rodriguez <[e-mail]>
Signed-off-by: Felix Kuehling <[e-mail]>
Acked-by: Oded Gabbay <[e-mail]>
Signed-off-by: Oded Gabbay <[e-mail]>

drivers/gpu/drm/amd/amdkfd/kfd_device.c

···
 	if (kfd->interrupts_active
 	    && interrupt_is_wanted(kfd, ih_ring_entry)
 	    && enqueue_ih_ring_entry(kfd, ih_ring_entry))
-		schedule_work(&kfd->interrupt_work);
+		queue_work(kfd->ih_wq, &kfd->interrupt_work);
 
 	spin_unlock(&kfd->interrupt_lock);
 }

drivers/gpu/drm/amd/amdkfd/kfd_interrupt.c

···
 		return r;
 	}
 
+	kfd->ih_wq = alloc_workqueue("KFD IH", WQ_HIGHPRI, 1);
 	spin_lock_init(&kfd->interrupt_lock);
 
 	INIT_WORK(&kfd->interrupt_work, interrupt_wq);
···
 	 * work-queue items that will access interrupt_ring. New work items
 	 * can't be created because we stopped interrupt handling above.
 	 */
-	flush_work(&kfd->interrupt_work);
+	flush_workqueue(kfd->ih_wq);
 
 	kfifo_free(&kfd->ih_fifo);
 }

drivers/gpu/drm/amd/amdkfd/kfd_priv.h

···
 
 	/* Interrupts */
 	struct kfifo ih_fifo;
+	struct workqueue_struct *ih_wq;
 	struct work_struct interrupt_work;
 	spinlock_t interrupt_lock;