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Re: [libvirt] [PATCH] libvirt: Change maximum CPU per node calculation.



On Wed, Oct 10, 2012 at 02:05:25PM +0200, Viktor Mihajlovski wrote:
> The macro VIR_NODEINFO_MAXCPUS computes the maximum number of supported
> CPUs for a node as the product of sockets, cores and threads from the
> virNodeInfo structure.
> Since topology cannot be discovered for offline CPUs, any of the values
> for sockets, cores or threads can be wrong (too small).
> As a consequence the per-VCPU CPU masks are truncated incorrectly and
> it's impossible to correlate the bits in the masks with actual CPUs on
> the host.
> 
> Example:
> Host has 3 logical CPUs, 1 socket, 3 cores, 1 thread.
> Guest has 1 virtual CPU and is started while all 3 host CPUs are
> online.
> 
> $ virsh vcpuinfo guest
> VCPU:           0
> CPU:            0
> State:          running
> CPU time:       35.4s
> CPU Affinity:   yyy
> 
> $ echo 0 > /sys/devices/system/cpu/cpu1/online
> 
> $ virsh vcpuinfo guest
> VCPU:           0
> CPU:            0
> State:          running
> CPU time:       35.5s
> CPU Affinity:   y-
> 
> The correct display for CPU affinity would have been y-y, as the guest
> continues to use CPUs 0 and 2.
> This is not a display problem only, because it is also not possible to
> explicitly pin the virtual CPU to host CPUs 0 and 2, due to the
> truncated CPU mask.
> 
> To prevent this, the following changes have been made:
> 
> 1. In virNodeParseNode count all logical CPUs, offline or online
>    and pass them back in a new argument 'maxcpus'.
> 2. Extend the virNodeInfo struct with a new member 'maxcpus'
>    containing the number of all logical CPUs, online and offline.
> 3. Change the macro VIR_NODEINFO_MAXCPUS to return virNodeInfo.maxpus
> 
> Signed-off-by: Viktor Mihajlovski <mihajlov linux vnet ibm com>
> ---
>  include/libvirt/libvirt.h.in |    3 ++-
>  src/nodeinfo.c               |   17 +++++++++++++----
>  src/remote/remote_protocol.x |    1 +
>  src/remote_protocol-structs  |    1 +
>  4 files changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/include/libvirt/libvirt.h.in b/include/libvirt/libvirt.h.in
> index 81f12a4..16265b5 100644
> --- a/include/libvirt/libvirt.h.in
> +++ b/include/libvirt/libvirt.h.in
> @@ -445,6 +445,7 @@ struct _virNodeInfo {
>                              total number of CPU sockets otherwise */
>      unsigned int cores; /* number of cores per socket */
>      unsigned int threads;/* number of threads per core */
> +    unsigned int maxcpus;/* number of maximum cpus - online and offline */
>  };

NACK, this breaks ABI compatibility of the public API


> diff --git a/src/remote/remote_protocol.x b/src/remote/remote_protocol.x
> index b0b530c..40883d0 100644
> --- a/src/remote/remote_protocol.x
> +++ b/src/remote/remote_protocol.x
> @@ -465,6 +465,7 @@ struct remote_node_get_info_ret { /* insert 1 */
>      int sockets;
>      int cores;
>      int threads;
> +    int maxcpus;
>  };
>  


NACK, this breaks ABI compatibility of the wire protocol

Daniel
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