After hot-adding CPUs to a Linux VM, few CPUs are offline.

Products

• VMware vSphere ESXi

Issue/Introduction

If a large amount of CPUs and memory is added to a VM in a single add operation, some of the CPUs may fail to be brought online automatically.

Output from the command, 'lscpu' will show the added CPUs as present, but some of the CPUs may appear offline.

$> lscpu

CPU(s): 768
On-line CPU(s) list: 0-390,392-400,403-411,413,414,416-419,422-428,431-466,468-477,479,480,482-485,487-492,494-499,501,503-505,507-536,538-564,566-573,575,576,578-583,585,587-589,591-605,607-624,626-642,644,645,648,649,651,652,654-666,670-672,674,675,677,678,680-687,689,690,692-697,700-702,704,706-710,713,715,716,720-725,727-733,735-738,740,743-750,753,755,757-766

Off-line CPU(s) list: 391,401,402,412,415,420,421,429,430,467,478,481,486,493,500,502,506,537,565,574,577,584,586,590,606,625,643,646,647,650,653,667-669,673,676,679,688,691,698,699,703,705,711,712,714,717-719,726,734,739,741,742,751,752,754,756,767

The issue occurs more frequently when memory is also being added to the VM at the same time.

Environment

Linux VMs running on ESXi 8.0.3 or later releases

Resolution

To resolve the issue, manually bring the CPUs online using the command,

œ echo 1 > /sys/devices/system/cpu/cpuXXX/online
  XXX will be the cpu number of the CPU that is offline.

For eg.

$> echo 1 > /sys/devices/system/cpu/cpu391/online