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Bug#871608: linux-image-4.9.0-3-amd64: Linux kernel should handle decreasing cpu steal clock counter gracefully

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Control: severity -1 important
Control: tag -1 moreinfo

On Wed, 2017-08-09 at 22:46 +0200, Michael Lass wrote:
> Package: src:linux
> Version: 4.9.30-2+deb9u3
> Severity: normal
> Tags: patch
> 
> Dear Maintainer,
> 
> running Debian Stretch as a paravirtualized guest under Xen, the kernel
> obtains its cpu steal time counter from the virtualization host. On some
> hosts, occasionally a slight decrease in the cpu steal time is returned
> which leads to an overflow of unsigned variables in the kernel and
> subsequent errors in steal time accounting (such as backwards running
> counters). This renders tools like "top" or "vmstat" broken in a way
> that the cpu utilization cannot be determined anymore.
> 
> While this is likely a bug in the virtualization environment, the kernel
> running as a guest should deal with this gracefully. I attached a patch
> to this report which fixes the errors caused by this on the guest.
> Kernel versions 4.7 and older, as well as 4.11 and newer should not be
> affected by this issue.
[...]

I agree that the kernel ought to work around this, but I'm hesitant to
add a fix that doesn't look like any upstream change.  Why and how do
you think this was fixed in 4.11?

Ben.

-- 
Ben Hutchings
If the facts do not conform to your theory, they must be disposed of.

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Bug#871608: linux-image-4.9.0-3-amd64: Linux kernel should handle decreasing cpu steal clock counter gracefully Ben Hutchings <ben@decadent.org.uk> - 2017-09-25 02:10 +0200
  Bug#871608: linux-image-4.9.0-3-amd64: Linux kernel should handle decreasing cpu steal clock counter gracefully Michael Laß <bevan@bi-co.net> - 2017-09-25 09:40 +0200
  Bug#871608: linux-image-4.9.0-3-amd64: Linux kernel should handle decreasing cpu steal clock counter gracefully Michael Laß <bevan@bi-co.net> - 2017-10-13 00:40 +0200

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