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[Tim Gardner <tim.gardner@xxxxxxxxxxxxx>]

** Also affects: linux (Ubuntu Yakkety)
   Importance: Medium
       Status: Confirmed

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https://bugs.launchpad.net/bugs/1579205

Title:
  CONFIG_NR_CPUS=256 is too low

Status in linux package in Ubuntu:
  Confirmed
Status in linux source package in Yakkety:
  Confirmed

Bug description:
  Intel Knights Landing systems boot up to 272 processor cores. The
  stock Ubuntu Linux kernel images leave processor cores above 256
  disabled:

  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 256/0xeb ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 257/0xef ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 258/0xf3 ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 259/0xf7 ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 260/0xfb ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 261/0xff ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 262/0x103 ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 263/0x107 ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 264/0x10b ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 265/0x10f ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 266/0x113 ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 267/0x117 ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 268/0x11b ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 269/0x11f ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 270/0x123 ignored.
  [    0.000000] ACPI: NR_CPUS/possible_cpus limit of 256 reached.  Processor 271/0x127 ignored.

  I'd request an increase in CONFIG_NR_CPUS to 512. This would match
  Debian's default setting. For comparison, RHEL/CentOS ship with 4096.

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