Commit c7275ce6 authored May 25, 2023 by Uday Shankar Committed by Keith Busch May 30, 2023

nvme: improve handling of long keep alives



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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>

parent 774a9636

drivers/nvme/host/core.c

+15 −1

Original line number	Diff line number	Diff line
		@@ -1199,6 +1199,20 @@ 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 - (rq->deadline - rq->timeout);
		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 {
		dev_warn(ctrl->device, "long keepalive RTT (%u ms)\n",
		jiffies_to_msecs(rtt));
		delay = 0;
		}

		blk_mq_free_request(rq);

		@@ -1217,7 +1231,7 @@ 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;
		}