[PATCH AUTOSEL 5.15 09/10] nvme: double KA polling frequency to avoid KATO with TBKAS on

Sasha Levin
2023-06-15T11:43:25Z

From: Uday Shankar <ushankar@purestorage.com>

[ Upstream commit ea4d453b9ec9ea279c39744cd0ecb47ef48ede35 ]

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>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/nvme/host/core.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index e5318b38c6624..98a7649a0f061 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1247,9 +1247,25 @@ EXPORT_SYMBOL_NS_GPL(nvme_execute_passthru_rq, NVME_TARGET_PASSTHRU);
* The host should send Keep Alive commands at half of the Keep Alive Timeout
* accounting for transport roundtrip times [..].
*/
+static unsigned long nvme_keep_alive_work_period(struct nvme_ctrl *ctrl)
+{
+ unsigned long delay = ctrl->kato * HZ / 2;
+
+ /*
+ * When using Traffic Based Keep Alive, we need to run
+ * nvme_keep_alive_work at twice the normal frequency, as one
+ * command completion can postpone sending a keep alive command
+ * by up to twice the delay between runs.
+ */
+ if (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS)
+ delay /= 2;
+ return delay;
+}
+
static void nvme_queue_keep_alive_work(struct nvme_ctrl *ctrl)
{
- queue_delayed_work(nvme_wq, &ctrl->ka_work, ctrl->kato * HZ / 2);
+ queue_delayed_work(nvme_wq, &ctrl->ka_work,
+ nvme_keep_alive_work_period(ctrl));
}

static void nvme_keep_alive_end_io(struct request *rq, blk_status_t status)
--
2.39.2