On Wed, 2024-05-22 at 15:41 +0530, Gk Gk wrote:
> Hi,
>
> If we create pinned vms, is there a chance of cpu steal in this scenario ?
yes, libvirt and by extention nova does not isolate kernel or operating system
processes from running on the cpu_dedicated_set cpus.
> If so what could be the reason since we have dedicated cpu assigned to the
> guest ?
kernel threads, os processes or irqs can all "steal" time from the vm
to prevent this you should adjust the default core mask in systemd for all operating
system userland process using

[Manager]
CPUAffinity=

in /etc/systemd/system.conf

addtionally you shoudl ensure that if irq balance is isntalled that it is confirured to not
balance irqs to the cpu_dedicated_set

i hesitate to mention the third option as i strongly recommend agaisnt using it but
if you want to be extra sure that host process dont steal you can us the kernel
isolcpus parmater. however that should reallly only be done on a realtime host and should
never contain any cores form the cpu_shared_set. if you add cores to isolcpus form thet cpu_dedicated_set
you need to ensure that cpu_shared_set is also defiend and your flavor have hw:emulator_thread_policy=shared
if you dont do that your vms may take 10 of minutes to boot and be unreliable and we will be talkign about
that problem in a week or so :)

i mention that the kernel parmater exists but unless you do your reasearch and understand what it
does you shoudl not use it.
>  What would happen if it is a physical cpu, directly assigned to the
> vm ? Does it experience cpu steal in this case as well ?
yes it would be the same. the other souce of cpu steal is the qemu emulator thread
that is why we recommend using hw:emulator_thread_policy=shared for all vms using cpu pinning
and have 1-2 cores in cpu_shared_set even if the host is going to used exclustively for pinned vms
if you are going to use it exlucsively for pinned vms you should also adjust the host reserved cpu config
option so that no other vms can boot on the cpu_shared_set and they are only used for the qemu emulator thread
the emulator thread is used for disk io and networking emulation as well as the monitor process that is used
ot recives commands form nova/libivrt. it is often mostly idel but it does run on the same cores as the vms
cpus so it steals tiem form the vms cpus by default.cpu_shared_set + hw:emulator_thread_policy=shared moves the emulator
thread ot other cores to aoivd that stealing.
>
> Thanks
> GK