Fedora 19 - does register hotplugged cpu1 Fedora-19 shows: /sys/devices/system/cpu/cpu1\ BUT NOT: /sys/devices/virtual/cpuid/cpu1\ strace udevadm --debug monitor ###when adding cpu1 recvmsg(5, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000001}, msg_iov(1)=[{"add@/devices/system/cpu/cpu1\0ACT"..., 8192}], msg_controllen=32, {cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=0, uid=0, gid=0}}, msg_flags=0}, 0) = 425 clock_gettime(CLOCK_MONOTONIC, {238115, 687196498}) = 0 write(1, "KERNEL[238115.687196] add /"..., 62) = 62 Fedora-18 shows: /sys/devices/system/cpu/cpu1\ /sys/devices/virtual/cpuid/cpu1\ strace udevadm --debug monitor ###when adding cpu1 recvmsg(5, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000001}, msg_iov(1)=[{"add@/devices/system/cpu/cpu1\0ACT"..., 8192}], msg_controllen=32, {cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=0, uid=0, gid=0}}, msg_flags=0}, 0) = 425 clock_gettime(CLOCK_MONOTONIC, {247341, 263395029}) = 0 write(1, "KERNEL[247341.263395] add /"..., 62) = 62 epoll_wait(3, {?} 0x7fff5cfd59b0, 4, -1) = 2 recvmsg(5, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000001}, msg_iov(1)=[{"add@/devices/virtual/cpuid/cpu1\0"..., 8192}], msg_controllen=32, {cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=0, uid=0, gid=0}}, msg_flags=0}, 0) = 145 clock_gettime(CLOCK_MONOTONIC, {247341, 267196621}) = 0 Kernel configuration CONFIG_HOTPLUG=y CONFIG_MEMORY_HOTPLUG=y CONFIG_MEMORY_HOTPLUG_SPARSE=y CONFIG_HOTPLUG_CPU=y CONFIG_BOOTPARAM_HOTPLUG_CPU0=y CONFIG_DEBUG_HOTPLUG_CPU0=y CONFIG_ARCH_ENABLE_MEMORY_HOTPLUG=y CONFIG_ACPI_HOTPLUG_CPU=y CONFIG_ACPI_HOTPLUG_MEMORY=y CONFIG_HOTPLUG_PCI_PCIE=y CONFIG_HOTPLUG_PCI=y CONFIG_HOTPLUG_PCI_ACPI=y CONFIG_HOTPLUG_PCI_ACPI_IBM=m # CONFIG_HOTPLUG_PCI_CPCI is not set CONFIG_HOTPLUG_PCI_SHPC=y # CONFIG_XEN_BALLOON_MEMORY_HOTPLUG is not set Test Case: console1: strace -o process_dump -ff udevadm --debug monitor console2: udevadm trigger --verbose --sysname-match=cpu1 --action=add Use Case: These are KVM hotplugged CPU's running in a virtual machine. The CPU's are hot-added.
/dev/cpu/1 missing
Including the rules below fixed the issue. Udev should be updated to include these rules in 5-0default-rules KERNEL="cpu[0-9]*", NAME="cpu/%n/cpuid" KERNEL="msr[0-9]*", NAME="cpu/%n/msr" KERNEL="microcode", NAME="cpu/microcode"