My system fails to detect new CPUs during CPU hot add process

Solution In Progress - Updated -

Environment

  • Red Hat Enterprise Linux 6 and 7
  • Systems capable of "hot adding" processors

Issue

  • When attempting to hot add a CPU to my system, the new processor is not detected and no new processor entries are seen in /sys/device/system/cpu/cpuXXX.

Resolution

  • Platforms that contain CPUs with the APIC ID of 255 or greater assigned require x2APIC enabled
  • Platforms that contain CPUs with the APIC ID of 255 or greater assigned may hit a bug in x2apic handling code. As a result, CPU hot add operation fails on RHEL7.1 and RHEL6.6. Please refer to the condition stated in "Root Cause" section.

Root Cause

  • Regardless of socket/core configuration, the hot add operation for the CPUs with APIC ID of 254 or less functions as expected. Whereas the hot add operation for the CPUs with APIC ID of 255 or more fails.
  • For example, when 3 socket-id or less are utilized the XXX below is always "0xx", whereas 4 socket-id onward it is "1xx", which requires the x2apic feature. Refer to below the legend as a reference:
    MSB LSB
    XXXYYYYYZ
    X = socket ID
    Y = core ID
    Z = thread ID
  • It is recommended for the users to consult the hardware vendor on whether the system has more than 256 logical processors and utilizes the x2apic function.
  • The feature is planned to be supported in 6.7 and 7.2.

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