[PATCH 1/3] nvme: double KA polling frequency to avoid KATO with TBKAS on

[Uday Shankar <ushankar@purestorage.com>]

With TBKAS on, the completion of one command can defer sending a
keep alive for up to twice the delay between successive runs of
nvme_keep_alive_work. The current delay of KATO / 2 thus makes it
possible for one command to defer sending a keep alive for up to
KATO, which can result in the controller detecting a KATO. The following
trace demonstrates the issue, taking KATO = 8 for simplicity:

1. t = 0: run nvme_keep_alive_work, no keep-alive sent
2. t = ε: I/O completion seen, set comp_seen = true
3. t = 4: run nvme_keep_alive_work, see comp_seen == true,
          skip sending keep-alive, set comp_seen = false
4. t = 8: run nvme_keep_alive_work, see comp_seen == false,
          send a keep-alive command.

Here, there is a delay of 8 - ε between receiving a command completion
and sending the next command. With ε small, the controller is likely to
detect a keep alive timeout.

Fix this by running nvme_keep_alive_work with a delay of KATO / 4
whenever TBKAS is on. Going through the above trace now gives us a
worst-case delay of 4 - ε, which is in line with the recommendation of
sending a command every KATO / 2 in the NVMe specification.

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 | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 6c1e7d6709e0..1298c7b9bffb 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1150,10 +1150,16 @@ EXPORT_SYMBOL_NS_GPL(nvme_passthru_end, NVME_TARGET_PASSTHRU);
  * 
  *   The host should send Keep Alive commands at half of the Keep Alive Timeout
  *   accounting for transport roundtrip times [..].
+ * 
+ * When TBKAS is on, we need to run nvme_keep_alive_work at twice this
+ * frequency, as one command completion can postpone sending a keep alive
+ * command by up to twice the delay between runs.
  */
 static void nvme_queue_keep_alive_work(struct nvme_ctrl *ctrl)
 {
-	queue_delayed_work(nvme_wq, &ctrl->ka_work, ctrl->kato * HZ / 2);
+	unsigned long delay = (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) ?
+		ctrl->kato * HZ / 4 : ctrl->kato * HZ / 2;
+	queue_delayed_work(nvme_wq, &ctrl->ka_work, delay);
 }
 
 static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
-- 
2.25.1