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: 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>
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