[PATCH 3/3] nvme: improve handling of long keep alives

[Uday Shankar <ushankar@purestorage.com>]

Upon keep alive completion, nvme_keep_alive_work is scheduled with the
same delay every time. If keep alive commands are completing slowly,
this may cause a keep alive timeout. The following trace illustrates the
issue, taking KATO = 8 and TBKAS off for simplicity:

1. t = 0: run nvme_keep_alive_work, send keep alive
2. t = ε: keep alive reaches controller, controller restarts its keep
          alive timer
3. t = 4: host receives keep alive completion, schedules
          nvme_keep_alive_work with delay 4
4. t = 8: run nvme_keep_alive_work, send keep alive

Here, a keep alive having RTT of 4 causes a delay of at least 8 - ε
between the controller receiving successive keep alives. With ε small,
the controller is likely to detect a keep alive timeout.

Fix this by calculating the RTT of the keep alive command, and adjusting
the scheduling delay of the next keep alive work accordingly.

Reported-by: Costa Sapuntzakis <costa@purestorage.com>
Reported-by: Randy Jennings <randyj@purestorage.com>
Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
---
 drivers/nvme/host/core.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 8a63051d7b5e..fbb8b2f41fe4 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1162,10 +1162,15 @@ EXPORT_SYMBOL_NS_GPL(nvme_passthru_end, NVME_TARGET_PASSTHRU);
  * frequency, as one command completion can postpone sending a keep alive
  * command by up to twice the delay between runs.
  */
+static unsigned long nvme_keep_alive_work_period(struct nvme_ctrl *ctrl)
+{
+	return (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) ?
+		(ctrl->kato * HZ / 4) : (ctrl->kato * HZ / 2);
+}
+
 static void nvme_queue_keep_alive_work(struct nvme_ctrl *ctrl)
 {
-	unsigned long delay = (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) ?
-		ctrl->kato * HZ / 4 : ctrl->kato * HZ / 2;
+	unsigned long delay = nvme_keep_alive_work_period(ctrl);
 	queue_delayed_work(nvme_wq, &ctrl->ka_work, delay);
 }
 
@@ -1175,6 +1180,15 @@ static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
 	struct nvme_ctrl *ctrl = rq->end_io_data;
 	unsigned long flags;
 	bool startka = false;
+	unsigned long rtt = jiffies - nvme_req(rq)->start_time;
+	unsigned long delay = nvme_keep_alive_work_period(ctrl);
+
+	/* Subtract off the keepalive RTT so nvme_keep_alive_work runs
+	 * at the desired frequency. */
+	if (rtt <= delay)
+		delay -= rtt;
+	else
+		delay = 0;
 
 	blk_mq_free_request(rq);
 
@@ -1194,7 +1208,7 @@ static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
 		startka = true;
 	spin_unlock_irqrestore(&ctrl->lock, flags);
 	if (startka)
-		nvme_queue_keep_alive_work(ctrl);
+		queue_delayed_work(nvme_wq, &ctrl->ka_work, delay);
 	return RQ_END_IO_NONE;
 }
 
-- 
2.25.1