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 f502e032e7e46..50a83306bea7b 100644 --- a/drivers/nvme/host/core.c +++ b/drivers/nvme/host/core.c @@ -1198,9 +1198,25 @@ 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 enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,