[SRU X] [PATCH 0/1] NVMe polling on timeout

Guilherme G. Piccoli gpiccoli at canonical.com
Fri Dec 7 21:28:10 UTC 2018

BugLink: https://launchpad.net/bugs/1807393


* NVMe controllers potentially could miss to send an interrupt, specially
due to bugs in virtual devices(which are common those days - qemu has its
own NVMe virtual device, so does AWS). This would be a difficult to
debug situation, because NVMe driver only reports the request timeout,
not the reason.

* The upstream patch proposed to SRU here was designed to provide more
information in these cases, by pro-actively polling the CQEs on request
timeouts, to check if the specific request was completed and some issue
(probably a missed interrupt) prevented the driver to notice, or if the
request really wasn't completed, which indicates more severe issues.

* Although quite useful for debugging, this patch could help to mitigate
issues in cloud environments like AWS, in case we may have jitter in
request completion and the i/o timeout was set to low values, or even
in case of atypical bugs in the virtual NVMe controller. With this patch,
if polling succeeds the NVMe driver will continue working instead of
trying a reset controller procedure, which may lead to fails in the
rootfs - refer to https://launchpad.net/bugs/1788035.

[Test Case]

* It's a bit tricky to artificially create a situation of missed
interrupt; one idea was to implement a small hack in the NVMe qemu
virtual device that given a trigger in guest kernel, will induce the
virtual device to skip an interrupt. The hack patch is present in a
Launchpad comment, along with instructions to reproduce.

[Regression Potential]

* There are no clear risks in adding such polling mechanism to the NVMe driver;
one bad thing that was neverreported but could happen with this patch is the
device could be in a bad state IRQ-wise that a reset would fix, but
the patch could cause all requests to be completed via polling, which
prevents the adapter reset. This is however a very hypothetical situation,
which would also happen in the mainline kernel (since it has the patch).

Keith Busch (1):
  nvme/pci: Poll CQ on timeout

 drivers/nvme/host/pci.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)


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