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(docs) Describe the flows of setting NUMA node affinity in Xen by Xenopsd
Signed-off-by: Bernhard Kaindl <[email protected]>
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‎doc/content/lib/xenctrl/xc_domain_node_setaffinity.md

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---
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title: xc_domain_node_setaffinity()
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description: Set a Xen domain's NUMA node affinity
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description: Set a Xen domain's NUMA node affinity for memory allocations
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mermaid:
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force: true
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---
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`xc_domain_node_setaffinity()` controls the NUMA node affinity of a domain.
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`xc_domain_node_setaffinity()` controls the NUMA node affinity of a domain,
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but it only updates the Xen hypervisor domain's `d->node_affinity` mask.
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This mask is read by the Xen memory allocator as the 2nd preference for the
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NUMA node to allocate memory from for this domain.
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By default, Xen enables the `auto_node_affinity` feature flag,
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where setting the vCPU affinity also sets the NUMA node affinity for
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memory allocations to be aligned with the vCPU affinity of the domain.
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> [!info] Preferences of the Xen memory allocator:
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> 1. A NUMA node passed to the allocator directly takes precedence, if present.
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> 2. Then, if the allocation is for a domain, it's `node_affinity` mask is tried.
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> 3. Finally, it falls back to spread the pages over all remaining NUMA nodes.
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As this call has no practical effect on the Xen scheduler, vCPU affinities
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need to be set separately anyways.
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The domain's `auto_node_affinity` flag is enabled by default by Xen. This means
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that when setting vCPU affinities, Xen updates the `d->node_affinity` mask
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to consist of the NUMA nodes to which its vCPUs have affinity to.
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See [xc_vcpu_setaffinity()](xc_vcpu_setaffinity) for more information
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on how `d->auto_node_affinity` is used to set the NUMA node affinity.
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Thus, so far, there is no obvious need to call `xc_domain_node_setaffinity()`
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when building a domain.
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Setting the NUMA node affinity using this call can be used,
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for example, when there might not be enough memory on the
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This function implements the functionality of `xc_domain_node_setaffinity`
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to set the NUMA affinity of a domain as described above.
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If the new_affinity does not intersect the `node_online_map`,
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it returns `-EINVAL`, otherwise on success `0`.
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it returns `-EINVAL`. Otherwise, the result is a success, and it returns `0`.
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When the `new_affinity` is a specific set of NUMA nodes, it updates the NUMA
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`node_affinity` of the domain to these nodes and disables `auto_node_affinity`
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for this domain. It also notifies the Xen scheduler of the change.
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`node_affinity` of the domain to these nodes and disables `d->auto_node_affinity`
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for this domain. With `d->auto_node_affinity` disabled,
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[xc_vcpu_setaffinity()](xc_vcpu_setaffinity) no longer updates the NUMA affinity
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of this domain.
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If `new_affinity` has all bits set, it re-enables the `d->auto_node_affinity`
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for this domain and calls
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[domain_update_node_aff()](https://github.com/xen-project/xen/blob/e16acd80/xen/common/sched/core.c#L1809-L1876)
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to re-set the domain's `node_affinity` mask to the NUMA nodes of the current
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the hard and soft affinity of the domain's online vCPUs.
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### Flowchart in relation to xc_set_vcpu_affinity()
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The effect of `domain_set_node_affinity()` can be seen more clearly on this
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flowchart which shows how `xc_set_vcpu_affinity()` is currently used to set
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the NUMA affinity of a new domain, but also shows how `domain_set_node_affinity()`
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relates to it:
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This sets the preference the memory allocator to the new NUMA nodes,
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and in theory, it could also alter the behaviour of the scheduler.
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This of course depends on the scheduler and its configuration.
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{{% include "xc_vcpu_setaffinity-xenopsd-notes.md" %}}
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{{% include "xc_vcpu_setaffinity-xenopsd.md" %}}
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`xc_domain_node_setaffinity` can be used to set the domain's `node_affinity`
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(which is normally set by `xc_set_vcpu_affinity`) to different NUMA nodes.
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#### No effect on the Xen scheduler
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Currently, the node affinity does not affect the Xen scheudler:
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In case `d->node_affinity` would be set before vCPU creation, the initial pCPU
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of the new vCPU is the first pCPU of the first NUMA node in the domain's
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`node_affinity`. This is further changed when one of more `cpupools` are set up.
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As this is only the initial pCPU of the vCPU, this alone does not change the
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scheduling of Xen Credit scheduler as it reschedules the vCPUs to other pCPUs.
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## Notes on future design improvements
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### It may be possible to call it before vCPUs are created
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When done early, before vCPU creation, some domain-related data structures
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could be allocated using the domain's `d->node_affinity` NUMA node mask.
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With further changes in Xen and `xenopsd`, Xen could allocate the vCPU structs
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on the affine NUMA nodes of the domain.
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For this, would be that `xenopsd` would have to call `xc_domain_node_setaffinity()`
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before vCPU creation, after having decided the domain's NUMA placement,
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preferably including claiming the required memory for the domain to ensure
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that the domain will be populated from the same NUMA node(s).
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This call cannot influence the past: The `xenopsd`
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[VM_create](../../xenopsd/walkthroughs/VM.start.md#2-create-a-xen-domain)
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micro-ops calls `Xenctrl.domain_create`. It currently creates

‎doc/content/lib/xenctrl/xc_vcpu_setaffinity-simplified.md

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---
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title: Simplified flowchart of xc_vcpu_setaffinity()
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description: See lib/xenctrl/xc_vcpu_setaffinity-xenopsd.md for an extended version
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hidden: true
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---
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```mermaid
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flowchart TD
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subgraph libxenctrl
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xc_vcpu_setaffinity("<tt>xc_vcpu_setaffinity()")--hypercall-->xen
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end
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subgraph xen[Xen Hypervisor]
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direction LR
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vcpu_set_affinity("<tt>vcpu_set_affinity()</tt><br>set the vCPU affinity")
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-->check_auto_node{"Is the domain's<br><tt>auto_node_affinity</tt><br>enabled?"}
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--"yes<br>(default)"-->
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auto_node_affinity("Set the<br>domain's<br><tt>node_affinity</tt>
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mask as well<br>(used for further<br>NUMA memory<br>allocation)")
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click xc_vcpu_setaffinity
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"https://github.com/xen-project/xen/blob/7cf16387/tools/libs/ctrl/xc_domain.c#L199-L250" _blank
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click vcpu_set_affinity
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"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1353-L1393" _blank
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click domain_update_node_aff
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"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1809-L1876" _blank
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click check_auto_node
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"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1840-L1870" _blank
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click auto_node_affinity
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"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1867-L1869" _blank
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end
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```

‎doc/content/lib/xenctrl/xc_vcpu_setaffinity-xenopsd-notes.md

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---
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title: Notes for the flowchart on the use of setaffinity for VM.start
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hidden: true
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---
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In the flowchart, two code paths are set in bold:
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- Show the path when `Host.numa_affinity_policy` is the default (off) in `xenopsd`.
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- Show the default path of `xc_vcpu_setaffinity(XEN_VCPUAFFINITY_SOFT)` in Xen,
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when the Domain's `auto_node_affinity` flag is enabled (default) to show
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how it changes to the vCPU affinity update the domain's `node_affinity`
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in this default case as well.
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[xenguest](../../xenopsd/walkthroughs/VM.build/xenguest/) uses the Xenstore
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to read the static domain configuration that it needs reads to build the domain.

‎doc/content/lib/xenctrl/xc_vcpu_setaffinity-xenopsd.md

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---
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title: Flowchart of the use of xc_vcpu_setaffinity() by xenopsd
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description: Shows how xenopsd uses xc_vcpu_setaffinity() to set NUMA affinity
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hidden: true
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---
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```mermaid
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flowchart TD
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subgraph VM.create["xenopsd VM.create"]
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%% Is xe vCPU-params:mask= set? If yes, write to Xenstore:
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is_xe_vCPUparams_mask_set?{"
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Is
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<tt>xe vCPU-params:mask=</tt>
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set? Example: <tt>1,2,3</tt>
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(Is used to enable vCPU<br>hard-affinity)
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"} --"yes"--> set_hard_affinity("Write hard-affinity to XenStore:
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<tt>platform/vcpu/#domid/affinity</tt>
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(xenguest will read this and other configuration data
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from Xenstore)")
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end
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subgraph VM.build["xenopsd VM.build"]
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%% Labels of the decision nodes
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is_Host.numa_affinity_policy_set?{
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Is<p><tt>Host.numa_affinity_policy</tt><p>set?}
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has_hard_affinity?{
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Is hard-affinity configured in <p><tt>platform/vcpu/#domid/affinity</tt>?}
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%% Connections from VM.create:
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set_hard_affinity --> is_Host.numa_affinity_policy_set?
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is_xe_vCPUparams_mask_set? == "no"==> is_Host.numa_affinity_policy_set?
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%% The Subgraph itself:
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%% Check Host.numa_affinity_policy
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is_Host.numa_affinity_policy_set?
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%% If Host.numa_affinity_policy is "best_effort":
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-- Host.numa_affinity_policy is<p><tt>best_effort -->
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%% If has_hard_affinity is set, skip numa_placement:
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has_hard_affinity?
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--"yes"-->exec_xenguest
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%% If has_hard_affinity is not set, run numa_placement:
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has_hard_affinity?
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--"no"-->numa_placement-->exec_xenguest
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%% If Host.numa_affinity_policy is off (default, for now),
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%% skip NUMA placement:
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is_Host.numa_affinity_policy_set?
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=="default: disabled"==>
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exec_xenguest
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end
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%% xenguest subgraph
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subgraph xenguest
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exec_xenguest
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==> stub_xc_hvm_build("<tt>stub_xc_hvm_build()")
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==> configure_vcpus("<tT>configure_vcpus()")
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%% Decision
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==> set_hard_affinity?{"
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Is <tt>platform/<br>vcpu/#domid/affinity</tt>
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set?"}
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end
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%% do_domctl Hypercalls
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numa_placement
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--Set the NUMA placement using soft-affinity-->
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XEN_VCPUAFFINITY_SOFT("<tt>xc_vcpu_setaffinity(SOFT)")
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==> do_domctl
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set_hard_affinity?
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--yes-->
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XEN_VCPUAFFINITY_HARD("<tt>xc_vcpu_setaffinity(HARD)")
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--> do_domctl
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xc_domain_node_setaffinity
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--Currently not used by the Xapi toolstack
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--> do_domctl
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%% Xen subgraph
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subgraph xen[Xen Hypervisor]
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subgraph domain_update_node_affinity["domain_update_node_affinity()"]
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domain_update_node_aff("<tt>domain_update_node_aff()")
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==> check_auto_node{"Is domain's<br><tt>auto_node_affinity</tt><br>enabled?"}
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=="yes (default)"==>set_node_affinity_from_vcpu_affinities("
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Calculate the domain's <tt>node_affinity</tt> mask from vCPU affinity
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(used for further NUMA memory allocation for the domain)")
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end
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do_domctl{"do_domctl()<br>op->cmd=?"}
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==XEN_DOMCTL_setvcpuaffinity==>
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vcpu_set_affinity("<tt>vcpu_set_affinity()</tt><br>set the vCPU affinity")
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==>domain_update_node_aff
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do_domctl
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--XEN_DOMCTL_setnodeaffinity (not used currently)
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-->is_new_affinity_all_nodes?
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subgraph domain_set_node_affinity["domain_set_node_affinity()"]
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is_new_affinity_all_nodes?{new_affinity<br>is #34;all#34;?}
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--is #34;all#34;
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--> enable_auto_node_affinity("<tt>auto_node_affinity=1")
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--> domain_update_node_aff
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is_new_affinity_all_nodes?
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--not #34;all#34;
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--> disable_auto_node_affinity("<tt>auto_node_affinity=0")
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--> domain_update_node_aff
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end
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%% setting and getting the struct domain's node_affinity:
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disable_auto_node_affinity
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--node_affinity=new_affinity-->
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domain_node_affinity
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set_node_affinity_from_vcpu_affinities
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==> domain_node_affinity@{ shape: bow-rect,label: "domain:&nbsp;node_affinity" }
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--XEN_DOMCTL_getnodeaffinity--> do_domctl
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end
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click is_Host.numa_affinity_policy_set?
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"https://github.com/xapi-project/xen-api/blob/90ef043c1f3a3bc20f1c5d3ccaaf6affadc07983/ocaml/xenopsd/xc/domain.ml#L951-L962"
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click numa_placement
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"https://github.com/xapi-project/xen-api/blob/90ef043c/ocaml/xenopsd/xc/domain.ml#L862-L897"
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click stub_xc_hvm_build
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"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L2329-L2436" _blank
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click get_flags
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"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1164-L1288" _blank
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click do_domctl
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"https://github.com/xen-project/xen/blob/7cf163879/xen/common/domctl.c#L282-L894" _blank
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click domain_set_node_affinity
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"https://github.com/xen-project/xen/blob/7cf163879/xen/common/domain.c#L943-L970" _blank
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click configure_vcpus
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"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1297-L1348" _blank
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click set_hard_affinity?
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"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1305-L1326" _blank
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click xc_vcpu_setaffinity
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"https://github.com/xen-project/xen/blob/7cf16387/tools/libs/ctrl/xc_domain.c#L199-L250" _blank
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click vcpu_set_affinity
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"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1353-L1393" _blank
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click domain_update_node_aff
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"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1809-L1876" _blank
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click check_auto_node
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"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1840-L1870" _blank
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click set_node_affinity_from_vcpu_affinities
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"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1867-L1869" _blank
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```

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