[Bug 1833660] Re: /lib/udev/rules.d/99-gce.rules tries to apply 'scheduler=none' to partitions

[kkm]

Sorry about messing with the status. I should have RTFM twice.

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Title:
  /lib/udev/rules.d/99-gce.rules tries to apply 'scheduler=none' to
  partitions

Status in gce-compute-image-packages package in Ubuntu:
  Fix Released
Status in gce-compute-image-packages source package in Xenial:
  Fix Committed
Status in gce-compute-image-packages source package in Bionic:
  Fix Committed
Status in gce-compute-image-packages source package in Disco:
  Fix Committed

Bug description:
  [Impact]

   * The following messages can be observed in the journal:

  systemd-udevd[1876]: sda14: Failed to open ATTR{/sys/devices/pci0000:00/0000:00:03.0/virtio0/host0/target0:0:1/0:0:1:0/block/sda/sda14/queue/scheduler} for writing: No such file or directory
   * This is not harmful, but annoying and does not show how we thrive to provide amazing experience with Ubuntu.

  [Test Case]

   * Run parted then exit:
     $ echo p | sudo parted
   * Observe the "Failed to open ATTR" messages not appearing in the journal in fixed versions.
     $ journalctl -a
     ...

  [Regression Potential]

   * Absolutely minimal, the fix just adds a filter to not change
  scheduler on partitions.

  
  [Original Bug Text]

  Xfer: https://github.com/GoogleCloudPlatform/compute-image-
  packages/issues/783

  I initially reported the bug there, but it appears the file is owned
  by you guys?

  I see this bug in Google Cloud images of 18.04 in
  --image=ubuntu-1804-bionic-v20190514 --image-project=gce-uefi-images.

  What happens is, the image contains the file
  /lib/udev/rules.d/99-gce.rules with the following rule:

  # Switch to using NOOP as the default scheduler per GCE request
  SUBSYSTEM=="block", ACTION=="add|change", ENV{ID_VENDOR}=="*Google*", ATTR{queue/scheduler}="noop"

  The rule matches both devices (/sda) and partitions (/sda1), but the
  scheduler is a device property and does not apply to partition. These
  lines are logged multiple times during the first boot of the image,
  when the partition and the filesystem is grown, and once on every
  subsequent boot, once per every partition:

      Jun  3 04:46:49 toy-sec-1 systemd-udevd[1442]: error opening ATTR{/sys/devices/pci0000:00/0000:00:03.0/virtio0/host0/target0:0:1/0:0:1:0/block/sda/sda1/queue/scheduler} for writing: No such file or directory
      Jun  3 04:46:49 toy-sec-1 systemd-udevd[1438]: error opening ATTR{/sys/devices/pci0000:00/0000:00:03.0/virtio0/host0/target0:0:1/0:0:1:0/block/sda/sda15/queue/scheduler} for writing: No such file or directory
      Jun  3 04:46:49 toy-sec-1 systemd-udevd[1437]: error opening ATTR{/sys/devices/pci0000:00/0000:00:03.0/virtio0/host0/target0:0:1/0:0:1:0/block/sda/sda14/queue/scheduler} for writing: No such file or directory

  To repro, no GCE necessary; you can boot any VM with the guest using
  the virtio driver, drop in this file, and run e. g. parted, or any
  program opening the raw device, as it triggers kernel uevents. Start
  parted, and the messages are logged. Quit parted, and they are logged
  again.

  This issue is harmless, but when you ingest logs, you'd rather have
  them as error-level message free as possible.

  I can think of 3 ways to solve this issue:

  1. Make the rule not match partitions. I drop-replace this file in all
  my images with the following:

      SUBSYSTEM=="block", ENV{DEVTYPE}!="partition",
  ACTION=="add|change", ENV{ID_VENDOR}=="*Google*",
  ATTR{queue/scheduler}="noop"

  2. Since Ubuntu is providing GCE images, kernel command line option
  'elevator=none' sets the I/O scheduler to all applicable devices by
  default; no udev integration necessary. The default is not locked, so
  if anyone needs to change it (e. g. for a physical disk directly
  attached to a VM, not a GCE setup but in a local VM it's possible),
  they can select a different elevator strategy with udev rules. This is
  the setting widely recommended by other Linux-based system, e. g.
  there is a RHEL support page recommending that. It obviously a better
  choice shift the I/O elevation job to the host, as it handles requests
  from all guests, and can prioritize I/O much better, as it has all
  consolidated information available at any moment for the physical
  device actually doing the block I/O.

  3. Since these GCE images come with a special kernel build (it has a '-gcp' version suffix), the default of none can be simply selected at compile time. [STRIKEOUT]It also make sense to compile in virtio into the kernel; as it is, the device is probed from initramfs. Since all VM boot drives are virtio, it is probably a sensible choice to have it compiled-in; definitely so for the GCP-specific kernel build.[/STRIKEOUT] EDIT: Sorry, just noticed virtio IS compiled
  into this kernel. But that was a side note besides the main point.

  Thanks, you'll probably know better than me which of these (or maybe
  other options I could not think of right now), as you probably
  understand all the implications I'm likely unaware of, so I'm just
  sharing my thoughts on this issue, not preferring any of these.

  I did not check other images available from the same GCE project, but
  I'm sure if the rule is there, the result will be identical--it's a
  kernel thing, and partitions do not have I/O schedulers by design.

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