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[William Grant <me@xxxxxxxxxxxxxxxxxx>]

I don't believe we've ever seen this in VMs. The main place excessive
kernel output has been problematic is on scalingstack compute nodes,
which are mostly HP ProLiant DL360p Gen8s (AMD64) and HP ProLiant m400s
(aka. mcdivitt, ARMv8). Both have a virtual serial port exposed by the
BMC or chassis, and kernel messages go to that. But the machines run VMs
-- they're not themselves virtualised.

In the DL360p case we get CPU hangs, while in the m400 case we get
excessive memory consumption.

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https://bugs.launchpad.net/bugs/1534216

Title:
  serial port driver in some virtual systems can hang

Status in linux package in Ubuntu:
  In Progress

Bug description:
  This is a clone from bug 1505564, to track the separate issue of the
  serial port driver failing to schedule itself off its cpu.

  The original bug's problem was caused by the kernel spamming a huge
  number of error messages in a certain situation.  Normally, that would
  not be a problem, but in this case the system is virtualized, and logs
  over its serial port.  When the massive number of kernel messages are
  sent to the serial port driver, it can't keep up, so sending all the
  log messages can take a very long time - minutes or longer - and the
  serial port driver fails to schedule itself off the cpu it's using
  during that time.  That results in other cpus hanging, waiting for the
  serial port driver's cpu to become avaiable.

  I'll  update the bug with more details as I debug.

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