Re: [PATCH v5 02/14] hw/core/machine: Introduce CPU cluster topology support

[Philippe Mathieu-Daudé]

Hi,

On 12/28/21 10:22, Yanan Wang wrote:
> The new Cluster-Aware Scheduling support has landed in Linux 5.16,
> which has been proved to benefit the scheduling performance (e.g.
> load balance and wake_affine strategy) on both x86_64 and AArch64.
> 
> So now in Linux 5.16 we have four-level arch-neutral CPU topology
> definition like below and a new scheduler level for clusters.
> struct cpu_topology {
>     int thread_id;
>     int core_id;
>     int cluster_id;
>     int package_id;
>     int llc_id;
>     cpumask_t thread_sibling;
>     cpumask_t core_sibling;
>     cpumask_t cluster_sibling;
>     cpumask_t llc_sibling;
> }
> 
> A cluster generally means a group of CPU cores which share L2 cache
> or other mid-level resources, and it is the shared resources that
> is used to improve scheduler's behavior. From the point of view of
> the size range, it's between CPU die and CPU core. For example, on
> some ARM64 Kunpeng servers, we have 6 clusters in each NUMA node,
> and 4 CPU cores in each cluster. The 4 CPU cores share a separate
> L2 cache and a L3 cache tag, which brings cache affinity advantage.
> 
> In virtualization, on the Hosts which have pClusters, if we can

Maybe [*] -> reference to pClusters?

> design a vCPU topology with cluster level for guest kernel and
> have a dedicated vCPU pinning. A Cluster-Aware Guest kernel can
> also make use of the cache affinity of CPU clusters to gain
> similar scheduling performance.
> 
> This patch adds infrastructure for CPU cluster level topology
> configuration and parsing, so that the user can specify cluster
> parameter if their machines support it.
> 
> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
> ---
>  hw/core/machine-smp.c | 26 +++++++++++++++++++-------
>  hw/core/machine.c     |  3 +++
>  include/hw/boards.h   |  6 +++++-
>  qapi/machine.json     |  5 ++++-
>  qemu-options.hx       |  7 ++++---
>  softmmu/vl.c          |  3 +++
>  6 files changed, 38 insertions(+), 12 deletions(-)

> diff --git a/qapi/machine.json b/qapi/machine.json
> index edeab6084b..ff0ab4ca20 100644
> --- a/qapi/machine.json
> +++ b/qapi/machine.json
> @@ -1404,7 +1404,9 @@
>  #
>  # @dies: number of dies per socket in the CPU topology
>  #
> -# @cores: number of cores per die in the CPU topology
> +# @clusters: number of clusters per die in the CPU topology

Missing:

   #            (since 7.0)

> +#
> +# @cores: number of cores per cluster in the CPU topology
>  #
>  # @threads: number of threads per core in the CPU topology
>  #
> @@ -1416,6 +1418,7 @@
>       '*cpus': 'int',
>       '*sockets': 'int',
>       '*dies': 'int',
> +     '*clusters': 'int',
>       '*cores': 'int',
>       '*threads': 'int',
>       '*maxcpus': 'int' } }
If you want I can update the doc when applying.

Thanks,

Phil.