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

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH v5 02/14] hw/core/machine: Introduce CPU cluster topology support
Date: Wed, 29 Dec 2021 16:44:30 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0

On 12/29/21 14:04, wangyanan (Y) wrote:
> On 2021/12/29 18:44, Philippe Mathieu-Daudé wrote:
>> On 12/29/21 04:48, wangyanan (Y) wrote:
>>> Hi Philippe,
>>> Thanks for your review.
>>> On 2021/12/29 3:17, Philippe Mathieu-Daudé wrote:
>>>> 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?
>>> Hm, I'm not sure what kind of reference is appropriate here.
>> So I guess the confusion comes from a simple typo =)
> I tried to mean "physical clusters" on host by pClusters, on the contrary
> to "virtual clusters" on guest. But obviously it brings confusion.

OK, I got confused because you don't use "vClusters".

>> Is it OK if I replace "pClusters" by "Clusters"?
> Sure, it's clearer to just use "clusters", please do that.


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