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>

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index dbfca1b925f8aa7ea7a57385281e1181f7a203d4..3ec38e2b91732691ae53ac50eb7fc8a45add5465 100644 (file)
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -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;
 }