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

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[ 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>
This commit is contained in:
Uday Shankar 2023-05-25 12:22:02 -06:00 committed by Greg Kroah-Hartman
parent c8f988c37a
commit 8a72260619
1 changed files with 17 additions and 1 deletions
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18
drivers/nvme/host/core.c
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@ -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,