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List:       kvm
Subject:    [Bug 191481] New: Virtual machine CPU counters are broken forever after live migration to system wit
From:       bugzilla-daemon () bugzilla ! kernel ! org
Date:       2016-12-29 16:21:38
Message-ID: bug-191481-28872 () https ! bugzilla ! kernel ! org/
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https://bugzilla.kernel.org/show_bug.cgi?id=191481

            Bug ID: 191481
           Summary: Virtual machine CPU counters are broken forever after
                    live migration to system with "steal time overflow"
                    KVM bug
           Product: Virtualization
           Version: unspecified
    Kernel Version: Found since 3.18.31, still resides in latest ubuntu
                    3.19..4.8 builds
          Hardware: Intel
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: kvm
          Assignee: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: ds@vo-ix.ru
        Regression: No

All (at least, compiled for ubuntu) kernels had the stolen time overflow bug,
described in links below:
http://lists.gnu.org/archive/html/qemu-devel/2015-06/msg01295.html
https://bugs.launchpad.net/linux/+bug/1494350

It was fixed (in ubuntu) on summer 2016, so till now still exists affected
hosts.

Kernels built before Apr 2016 bypass "%still" counter as is, so sar output
likes this:
11:11:15 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
11:10:48 AM     all      0.00      0.00      0.50      0.00      0.00     99.50
11:10:49 AM     all      0.00      0.00      0.00      0.00   
18823208238479134720.00    203.06
11:10:50 AM     all      0.00      0.00      0.00      0.00      0.00    100.00

While all newer kernel counters I've been tested sticks:
11:11:15 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
07:07:56 AM     all      0.50      0.00      0.00      0.00      0.00     99.50
07:07:57 AM     all      0.00      0.00      0.50      0.00      0.00     99.50
07:07:58 AM     all      0.00      0.00      0.50      0.00      0.00     99.50
07:07:59 AM     all      0.00      0.00      0.00      0.00    100.00      0.00
07:08:00 AM     all      0.00      0.00      0.00      0.00    100.00      0.00
07:08:01 AM     all      0.00      0.00      0.00      0.00    100.00      0.00

%steal is always 100% after migration.

I slightly suspect that the reason resides in commit
0185604c2d82c560dab2f2933a18f797e74ab5a8.

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