Commit ea4d453b authored by Uday Shankar's avatar Uday Shankar Committed by Keith Busch

nvme: double KA polling frequency to avoid KATO with TBKAS on

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: default avatarCosta Sapuntzakis <costa@purestorage.com>
Reported-by: default avatarRandy Jennings <randyj@purestorage.com>
Signed-off-by: default avatarUday Shankar <ushankar@purestorage.com>
Reviewed-by: default avatarHannes Reinecke <hare@suse.de>
Reviewed-by: default avatarSagi Grimberg <sagi@grimberg.me>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarKeith Busch <kbusch@kernel.org>
parent 31a59782
......@@ -1163,9 +1163,25 @@ 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 [..].
*/
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,
......
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