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[Chris Valean <cvalean@xxxxxxxxxxxxxxxxxxxxxx>]

Hi Joe,

I have a 15.04 install running linux-next 4.5.0-rc5 from Febr 26th, on
which I can confirm Sidney's update, that udev/systemd is no longer
slowing this down to a halt.

As the default 3.19 kernel on 15.04 is not having this behavior, I've installed kernel 4.2.8 from the ubuntu ppa mainline tree.
Running 4.2.8 on 15.04 brings us to the original problem with systemd-udevd CPU cycles.
It would be great if you can do the bisect this as high as kernel 4.4 from Xenial, as that one was also showing the same symptoms, so you can narrow down the patches between 4.4 (xenial) and 4.5 upstream.
Thank you!

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https://bugs.launchpad.net/bugs/1496927

Title:
  [Hyper-V] udev trying to modify owner and mode of /dev/vmbus/hv_vss

Status in linux package in Ubuntu:
  In Progress
Status in linux source package in Wily:
  In Progress
Status in linux source package in Xenial:
  In Progress

Bug description:
  While testing Ubuntu 15.10 Wily Werewolf (development branch) 64 bit
  we found the below issue.

  Repro rate: 100%
  Repro details:
  Hyper-V: Server 2012 (no R2) - WS 2012 is the only affected platform
  Daily build version: 2015.09.10
  Kernel: kernel 4.2.0-7-generic

  During the below event, we also noticed that system-udevd is
  constantly around 100% CPU on the VM, taking a lot of cycles.

  On Server 2012 we don't have the Live backup with VSS feature, so
  these messages should not occur in the first place.

  After 20 minutes we have over 3000 messages in the logs, all like below:
  Sep 14 17:43:33 ubuntu systemd-udevd[14292]: setting owner of /dev/vmbus/hv_vss to uid=0, gid=0 failed: No such file or directory
  Sep 14 17:44:00 ubuntu systemd-udevd[14292]: setting mode of /dev/vmbus/hv_vss to 020600 failed: No such file or directory
  Sep 14 17:44:00 ubuntu systemd-udevd[14292]: setting owner of /dev/vmbus/hv_vss to uid=0, gid=0 failed: No such file or directory
  Sep 14 17:44:01 ubuntu systemd-udevd[14292]: setting mode of /dev/vmbus/hv_vss to 020600 failed: No such file or directory

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