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[PE Norris <paul@xxxxxxxxxxx>]

I can't speak for the OP, but I was hit by this bug with an almost
identical CPU setup (Xeon x5355) and the kernel in proposed doesn't
resolve the issue for me.  The upstream utopic kernel link posted at #7
however does fix it.   My symptoms are identical.

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https://bugs.launchpad.net/bugs/1386473

Title:
  Not using all cores after upgrade to 14.10

Status in “linux” package in Ubuntu:
  Confirmed
Status in “linux” source package in Utopic:
  Confirmed

Bug description:
  I have a computer with two cpus with 4 cores each (physical) but htop
  is only showing activity on two of these 8 cores (number 1 and 3).
  Also, if I run "stress -c 8" top shows only 25% cpu usage. I've
  rebooted the machine and the problem persists. I also took a look at
  the BIOS settings and found nothing there that looked suspicious.

  If I run 'stress -c 8' only two cores go to 100% in htop as stated
  above (cores 1 and 3). If I run 'stress -c 1', one core goes to 100%
  (either core 1 or 3). If I run taskset -cp <coreid> <pid> I can
  control which core the process is run on. I can actually move it to
  any core that I want (1, 3 or any other one)! So the cores are
  there... the kernel just doesn't seem to use it unless I explicitly
  tell it to with taskset.

   I've verified that this is a regression when moving from [lxk]ubuntu
  14.04 to 14.10. I used pristine lubuntu 14.10 and 14.04 usb sticks to
  test this out and verified that a cpu sysbench test using 8 threads
  took ~15 sec on 14.04 but ~55 sec on 14.10. The computer this is
  happening on is a Dell Precision T5400 tower workstation.

  ProblemType: Bug
  DistroRelease: Ubuntu 14.10
  Package: linux-image-3.16.0-23-generic 3.16.0-23.31
  ProcVersionSignature: Ubuntu 3.16.0-23.31-generic 3.16.4
  Uname: Linux 3.16.0-23-generic x86_64
  NonfreeKernelModules: nvidia
  ApportVersion: 2.14.7-0ubuntu8
  Architecture: amd64
  CRDA: Error: [Errno 2] No such file or directory: 'iw'
  CurrentDesktop: KDE
  Date: Tue Oct 28 00:46:25 2014
  HibernationDevice: RESUME=UUID=20536d9f-ef05-46dd-a104-47367e464fcd
  InstallationDate: Installed on 2014-02-05 (264 days ago)
  InstallationMedia: Ubuntu-Server 13.10 "Saucy Salamander" - Release amd64 (20131016)
  IwConfig:
   eth0      no wireless extensions.
   
   lo        no wireless extensions.
  MachineType: Dell Inc. Precision WorkStation T5400
  ProcFB:
   
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.16.0-23-generic root=UUID=28fc4c4c-cecd-4abe-a788-28f69e745845 ro splash quiet
  RelatedPackageVersions:
   linux-restricted-modules-3.16.0-23-generic N/A
   linux-backports-modules-3.16.0-23-generic  N/A
   linux-firmware                             1.138
  RfKill:
   
  SourcePackage: linux
  UpgradeStatus: Upgraded to utopic on 2014-10-24 (3 days ago)
  dmi.bios.date: 02/01/2008
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: A01
  dmi.board.name: 0RW203
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A00
  dmi.chassis.type: 7
  dmi.chassis.vendor: Dell Inc.
  dmi.modalias: dmi:bvnDellInc.:bvrA01:bd02/01/2008:svnDellInc.:pnPrecisionWorkStationT5400:pvr:rvnDellInc.:rn0RW203:rvrA00:cvnDellInc.:ct7:cvr:
  dmi.product.name: Precision WorkStation T5400
  dmi.sys.vendor: Dell Inc.

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