Re: [Qemu-devel] [PATCH 0/3] add query-cpu-fast and related s390 changes

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 12 Feb 2018 13:14:29 +0100
Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com> wrote:

> This series consolidates patches around a performance issue
> caused by the usage of QMP query-cpus.

Thank you for consolidating this; it was a bit hard to follow the
different discussions.

> 
> A performance issue was found in an OpenStack environment, where
> ceilometer was collecting domain statistics with libvirt. The domain
> statistics reported by libvirt include the vCPU halted state, which 
> in turn is retrieved with QMP query-cpus.
> 
> This causes two issues:
> 1. Performance: on most architectures query-cpus needs to issue a KVM ioctl
>    to find out whether a vCPU was halted. This is not the case for s390
>    but query-cpus is always causing the vCPU to exit the VM.
> 
> 2. Semantics: on x86 and other architectures, halted is a highly transient
>    state, which is likely to have already changed shortly after the state
>    information has been retrieved. This is not the case for s390, where
>    halted is an indication that the vCPU is stopped, meaning its not
>    available to the guest operating system until it has been restarted.
> 
> The following patches help to alleviate the issues:
> 
> Patch 1/3:
>   Adds architecture specific data to the QMP CpuInfo type, exposing
>   the existing s390 cpu-state in QMP. The cpu-state is a representation
>   more adequate than the ambiguous 'halted' condition.
> 
>   Changes since original v2:
>   - fixed cpu-state usage in hw/intc/s390_flic.c, necessary because
>     master was updated in the meantime
>   - removed superfluous newline while printing cpu-state

So this is adding s390x info in query-cpus (presumably for legacy
callers?)...

> 
> Patch 2/3:
>   Adds a new QMP function query-cpus-fast, which will only retrieve
>   vCPU information that can be obtained without interrupting the
>   vCPUs of a running VM. It introduces a new return type CpuInfoFast
>   with the subset of fields meeting this condition. Specifically, the
>   halted state is not part of CpuInfoFast. QMP clients like libvirt
>   are encouraged to switch to the new API for vCPU information.
> 
>   Changes since original v2:
>   - dropped optional halted state from CpuInfoFast

...this is adding the new query-cpus-fast...

> 
> Patch 3/3:
>   Adds the s390-specific cpu state to CpuInfoFast, allowing management
>   apps to find out whether a vCPU is in the stopped state. This extension
>   leads to a partial duplication of field definitions from CpuInfo
>   to CpuInfoFast. This should be tolerable if CpuInfo is deprecated and
>   eventually removed.

...and this is adding the s390x state to query-cpus-fast, right?

> 
> Luiz Capitulino (1):
>   qmp: add query-cpus-fast
> 
> Viktor Mihajlovski (2):
>   qmp: expose s390-specific CPU info
>   qmp: add architecture specific cpu data for query-cpus-fast
> 
>  cpus.c                     |  54 ++++++++++++++++++++
>  hmp-commands-info.hx       |  14 ++++++
>  hmp.c                      |  33 +++++++++++++
>  hmp.h                      |   1 +
>  hw/intc/s390_flic.c        |   4 +-
>  hw/s390x/s390-virtio-ccw.c |   2 +-
>  qapi-schema.json           | 121 ++++++++++++++++++++++++++++++++++++++++++++-
>  target/s390x/cpu.c         |  24 ++++-----
>  target/s390x/cpu.h         |   7 +--
>  target/s390x/kvm.c         |   8 +--
>  target/s390x/sigp.c        |  38 +++++++-------
>  11 files changed, 262 insertions(+), 44 deletions(-)
>