Description of problem:

When running F19 (and, most likely, all previous versions - see below) as KVM guest, active agetty constantly consumes a bit of CPU (1% in my case), which is elevated to ca. 4-5% of host CPU usage. Given that most virtualized systems are running headless but will have at least one agetty running, several of them will consume significant amount of CPU time at the host even when idling.

Version-Release number of selected component (if applicable):

util-linux-2.23.1-2.fc19.x86_64

How reproducible:

100% (Proxmox 3.0 KVM, though it may affect any QEMU-KVM or even other VMs/hypervisors).

Steps to Reproduce:
1. Run F19 inside KVM
2. Login and logout on console (though in my case login is not necessary)
3. Make sure that the guest is idle
4. Observe CPU usage in host and guest.

Actual results:

Checking CPU usage reveals that agetty in guest system is constantly consuming ca. 1% of CPU time (may depend on CPU type, it is Core i5-3570T in my case), host system (KVM process) is consuming ca. 4-5% of CPU. Running 6 active agetty processes leads to ca. 12-13% CPU usage in host (each still consuming ca. 1% in guest).

Expected results:

CPU usage should be very close to 0% while idling (as it is when agetty is not running).

Additional info:

ltrace of running agetty reveals the following loop:

16:36:30.110604 usleep(1000)                                                                                  = <void>
16:36:30.112047 read(0 <no return ...>
error: maximum array length seems negative
, "\n", 1)                                                                                                    = -1
16:36:30.112494 __errno_location()                                                                            = 0x7f02ac6756c0
16:36:30.112829 usleep(1000)                                                                                  = <void>
16:36:30.114235 read(0 <no return ...>
error: maximum array length seems negative
, "\n", 1)                                                                                                    = -1
16:36:30.114748 __errno_location()                                                                            = 0x7f02ac6756c0

This loop (function get_logname in agetty.c) is the cause of elevated CPU usage, as it is repeated every 1ms - actually, just running usleep(1000) in a loop (without any I/O) will lead to this result.

Probably, this can (and should) be fixed outside of agetty (at hypervisor/VM/kernel level, as it is related to nanosleep implementation), but for the time being it makes sense to get rid of this loop in agetty to avoid unnecessary CPU consumption in virtualized environments.

PS: There are many reports about Linux KVM guests consuming up to 10% of host CPU when idle even on tickless kernels, so I believe that usleep() loop on agetty may be the cause.