[RFC PATCH 1/3] nvme/pci: Poll the cq in the primary irq handler

[Keith Busch <kbusch@kernel.org>]

The nvme threaded interrupt handler reduces CPU time spent in hard irq
context, but waking it increases latency for low queue depth workloads.

Poll the completion queue once from the primary handler and wake the
thread only if more completions remain after. Since there is a window
of time where the threaded and primary handlers may run simultaneously,
add a new nvmeq flag so that the two can synchronize which owns processing
the queue.

Signed-off-by: Keith Busch <kbusch@kernel.org>
---
 drivers/nvme/host/pci.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 4022a872d29c..c811c0984fe0 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -186,6 +186,7 @@ struct nvme_queue {
 #define NVMEQ_SQ_CMB		1
 #define NVMEQ_DELETE_ERROR	2
 #define NVMEQ_POLLED		3
+#define NVMEQ_THREAD		4
 	u32 *dbbuf_sq_db;
 	u32 *dbbuf_cq_db;
 	u32 *dbbuf_sq_ei;
@@ -1047,15 +1048,27 @@ static irqreturn_t nvme_irq_thread(int irq, void *data)
 		__pci_msix_desc_mask_irq(irq_get_msi_desc(irq), 0);
 	else
 		writel(1 << nvmeq->cq_vector, nvmeq->dev->bar + NVME_REG_INTMC);
+	clear_bit(NVMEQ_THREAD, &nvmeq->flags);
 	return IRQ_HANDLED;
 }
 
 static irqreturn_t nvme_irq_check(int irq, void *data)
 {
 	struct nvme_queue *nvmeq = data;
+	irqreturn_t ret;
 
 	if (!nvme_cqe_pending(nvmeq))
 		return IRQ_NONE;
+
+	if (test_and_set_bit(NVMEQ_THREAD, &nvmeq->flags))
+		return IRQ_NONE;
+
+	ret = nvme_irq(irq, data);
+	if (!nvme_cqe_pending(nvmeq)) {
+		clear_bit(NVMEQ_THREAD, &nvmeq->flags);
+		return ret;
+	}
+
 	if (to_pci_dev(nvmeq->dev->dev)->msix_enabled)
 		__pci_msix_desc_mask_irq(irq_get_msi_desc(irq), 1);
 	else
-- 
2.21.0


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