Stake a claim for further memory for a domain, and release it too.
Set a Xen domain's NUMA node affinity for memory allocations
Populate a Xen domain's physical memory map
Set a Xen vCPU's pCPU affinity and the domain's NUMA node affinity
The purpose of xc_domain_claim_pages() is to attempt to
stake a claim on an amount of memory for a given domain which guarantees that
memory allocations for the claimed amount will be successful.
The domain can still attempt to allocate beyond the claim, but those are not
guaranteed to be successful and will fail if the domain’s memory reaches it’s
max_mem value.
The aim is to attempt to stake a claim for a domain on a quantity of pages of system RAM, but not assign specific page frames. It performs only arithmetic so the hypercall is very fast and not be preempted. Thereby, it sidesteps any time-of-check-time-of-use races for memory allocation.
xc_domain_claim_pages() returns 0 if the Xen page allocator has atomically
and successfully claimed the requested number of pages, else non-zero.
Errors returned by xc_domain_claim_pages() must be handled as they are a
normal result of the xenopsd thread-pool claiming and starting many VMs
in parallel during a boot storm scenario.
This is especially important when staking claims on NUMA nodes using an updated version of this function. In this case, the only options of the calling worker thread would be to adapt to the NUMA boot storm: Attempt to find a different NUMA node for claiming the memory and try again.
For reference, see the hypercall comment Xen hypervisor header: xen/include/public/memory.h
The design of the backing hypercall in Xen is as follows:
To set up the boot memory of new domain, the libxenguest function xc_dom_boot_mem_init relies on this design:
The memory setup
of xenguest and by the xl CLI using libxl use it.
libxl actively uses it to stack an initial claim before allocating memory.
The functions
meminit_hvm()
and meminit_pv used by
xc_dom_boot_mem_init()
always destroy any remaining claim before they return.
Thus, after libxenguest has completed allocating and populating
the physical memory of the domain, no domain has a remaining claim!
From this point onwards, no memory allocation is guaranteed! While swapping memory between domains can be expected to always succeed, Allocating a new page after freeing another can always fail unless a privileged domain stakes a claim for such allocations beforehand!
The Xen upstream memory claims code is implemented to work as follows:
When allocating memory, while a domain has an outstanding claim, Xen updates remaining claim accordingly.
If a domain has a stake from claim, when memory is freed, freed amount of memory increases the stake of memory.
Xen doesn’t know if a page was allocated using a NUMA node claim. Therefore, it cannot know if it would be legal to increment a stake a NUMA node claim when freeing pages.
The smallest possible change to achieve a similar effect would be to add a field to the Xen hypervisor’s domain struct. It would be used for remembering the last total NUMA node claim. With it, freeing memory from a NUMA node could attempt to increase the outstanding claim on the claimed NUMA node, but only given this amount is available and not claimed.
xl claims reports the outstanding claims of the domains:xl claims:Name ID Mem VCPUs State Time(s) Claimed
Domain-0 0 2656 8 r----- 957418.2 0xl info reports the host-wide outstanding claims:xl info | grep outstanding:outstanding_claims : 0Xen only tracks:
Claiming zero pages effectively cancels the domain’s outstanding claim and is always successful.
max_mem value is used to deny memory allocationIf an allocation would cause the domain to exceed it’s max_mem
value, it will always fail.
Function signature of the libXenCtrl function to call the Xen hypercall:
long xc_memory_op(libxc_handle, XENMEM_claim_pages, struct xen_memory_reservation *)struct xen_memory_reservation is defined as :
struct xen_memory_reservation {
.nr_extents = nr_pages, /* number of pages to claim */
.extent_order = 0, /* an order 0 means: 4k pages, only 0 is allowed */
.mem_flags = 0, /* no flags, only 0 is allowed (at the moment) */
.domid = domid /* numerical domain ID of the domain */
};It is used by libxenguest, which is used at least by:
libxl/the xl CLI.The xl cli uses claims actively. By default, it enables libxl to pass
the struct xc_dom_image with its claim_enabled field set.
The functions dispatched by xc_boot_mem_init()
then attempt to claim the boot using xc_domain_claim_pages().
They also (unconditionally) destroy any open claim upon return.
This means that in case the claim fails, xl avoids:
With a
proposed update
of xc_domain_claim_pages() for NUMA node claims, a node argument is added.
It can be XC_NUMA_NO_NODE for defining a host-wide claim or a NUMA node
for staking a claim on one NUMA node.
This update does not change the foundational design of memory claims of the Xen hypervisor where a claim is defined as a single claim for the domain.
For improved support for NUMA, xenopsd
is updated to call an updated version of this function for the domain.
It reserves NUMA node memory before xenguest is called, and a new pnode
argument is added to xenguest.
It sets the NUMA node for memory allocations by the xenguest
boot memory setup
xc_boot_mem_init() which also
sets the exact flag.
The exact flag forces get_free_buddy() to fail
if it could not find scrubbed memory for the give pnode which causes
alloc_heap_pages()
to re-run get_free_buddy() with the flag to
fall back to dirty, not yet scrubbed memory.
alloc_heap_pages() then checks each 4k page for the need to scrub and scrubs those before returning them.
This is expected to improve the issue of potentially spreading memory over all NUMA nodes in case of parallel boot by preventing one NUMA node to become the target of parallel memory allocations which cannot fit on it and also the issue of spreading memory over all NUMA nodes if case of a huge amount of dirty memory that needs to be scrubbed for before a domain can start or restart.
This is not seen as an issue as grant tables and I/O pages are usually not as frequently used as regular system memory of domains, but this potential issue remains:
In discussions, it was said that Windows PV drivers unmap and free memory
for grant tables to Xen and then re-allocate memory for those grant tables.
xenopsd may want to try to stake a very small claim for the domain on the
NUMA node of the domain so that Xen can increase this claim when the PV drivers
free this memory and re-use the resulting claimed amount for allocating
the grant tables.
This would ensure that the grant tables are then allocated on the local NUMA node of the domain, avoiding remote memory accesses when accessing the grant tables from inside the domain.
Note: In case the corresponding backend process in Dom0 is running on another NUMA node, it would access the domain’s grant tables from a remote NUMA node, but in this would enable a future improvement for Dom0, where it could prefer to run the corresponding backend process on the same or a neighbouring NUMA node.
xc_domain_node_setaffinity() controls the NUMA node affinity of a domain,
but it only updates the Xen hypervisor domain’s d->node_affinity mask.
This mask is read by the Xen memory allocator as the 2nd preference for the
NUMA node to allocate memory from for this domain.
node_affinity mask is tried.As this call has no practical effect on the Xen scheduler, vCPU affinities need to be set separately anyways.
The domain’s auto_node_affinity flag is enabled by default by Xen. This means
that when setting vCPU affinities, Xen updates the d->node_affinity mask
to consist of the NUMA nodes to which its vCPUs have affinity to.
See xc_vcpu_setaffinity() for more information
on how d->auto_node_affinity is used to set the NUMA node affinity.
Thus, so far, there is no obvious need to call xc_domain_node_setaffinity()
when building a domain.
Setting the NUMA node affinity using this call can be used, for example, when there might not be enough memory on the preferred NUMA node, but there are other NUMA nodes that have enough free memory to be used for the system memory of the domain.
In terms of future NUMA design, it might be even more favourable to
have a strategy in xenguest where in such cases, the superpages
of the preferred node are used first and a fallback to neighbouring
NUMA nodes only happens to the extent necessary.
Likely, the future allocation strategy should be passed to xenguest
using Xenstore like the other platform parameters for the VM.
classDiagram
class `xc_domain_node_setaffinity()` {
+xch: xc_interface #42;
+domid: uint32_t
+nodemap: xc_nodemap_t
0(on success)
-EINVAL(if a node in the nodemask is not online)
}
click `xc_domain_node_setaffinity()` href "
https://github.com/xen-project/xen/blob/master/tools/libs/ctrl/xc_domain.c#L122-L158"
`xc_domain_node_setaffinity()` --> `Xen hypercall: do_domctl()`
`xc_domain_node_setaffinity()` <-- `Xen hypercall: do_domctl()`
class `Xen hypercall: do_domctl()` {
Calls domain_set_node_affinity#40;#41; and returns its return value
Passes: domain (struct domain *, looked up using the domid)
Passes: new_affinity (modemask, converted from xc_nodemap_t)
}
click `Xen hypercall: do_domctl()` href "
https://github.com/xen-project/xen/blob/master/xen/common/domctl.c#L516-L525"
`Xen hypercall: do_domctl()` --> `domain_set_node_affinity()`
`Xen hypercall: do_domctl()` <-- `domain_set_node_affinity()`
class `domain_set_node_affinity()` {
domain: struct domain
new_affinity: nodemask
0(on success, the domain's node_affinity is updated)
-EINVAL(if a node in the nodemask is not online)
}
click `domain_set_node_affinity()` href "
https://github.com/xen-project/xen/blob/master/xen/common/domain.c#L943-L970"This function implements the functionality of xc_domain_node_setaffinity
to set the NUMA affinity of a domain as described above.
If the new_affinity does not intersect the node_online_map,
it returns -EINVAL. Otherwise, the result is a success, and it returns 0.
When the new_affinity is a specific set of NUMA nodes, it updates the NUMA
node_affinity of the domain to these nodes and disables d->auto_node_affinity
for this domain. With d->auto_node_affinity disabled,
xc_vcpu_setaffinity() no longer updates the NUMA affinity
of this domain.
If new_affinity has all bits set, it re-enables the d->auto_node_affinity
for this domain and calls
domain_update_node_aff()
to re-set the domain’s node_affinity mask to the NUMA nodes of the current
the hard and soft affinity of the domain’s online vCPUs.
The effect of domain_set_node_affinity() can be seen more clearly on this
flowchart which shows how xc_set_vcpu_affinity() is currently used to set
the NUMA affinity of a new domain, but also shows how domain_set_node_affinity()
relates to it:
In the flowchart, two code paths are set in bold:
Host.numa_affinity_policy is the default (off) in xenopsd.xc_vcpu_setaffinity(XEN_VCPUAFFINITY_SOFT) in Xen,
when the Domain’s auto_node_affinity flag is enabled (default) to show
how it changes to the vCPU affinity update the domain’s node_affinity
in this default case as well.xenguest uses the Xenstore to read the static domain configuration that it needs reads to build the domain.
flowchart TD
subgraph VM.create["xenopsd VM.create"]
%% Is xe vCPU-params:mask= set? If yes, write to Xenstore:
is_xe_vCPUparams_mask_set?{"
Is
<tt>xe vCPU-params:mask=</tt>
set? Example: <tt>1,2,3</tt>
(Is used to enable vCPU<br>hard-affinity)
"} --"yes"--> set_hard_affinity("Write hard-affinity to XenStore:
<tt>platform/vcpu/#domid/affinity</tt>
(xenguest will read this and other configuration data
from Xenstore)")
end
subgraph VM.build["xenopsd VM.build"]
%% Labels of the decision nodes
is_Host.numa_affinity_policy_set?{
Is<p><tt>Host.numa_affinity_policy</tt><p>set?}
has_hard_affinity?{
Is hard-affinity configured in <p><tt>platform/vcpu/#domid/affinity</tt>?}
%% Connections from VM.create:
set_hard_affinity --> is_Host.numa_affinity_policy_set?
is_xe_vCPUparams_mask_set? == "no"==> is_Host.numa_affinity_policy_set?
%% The Subgraph itself:
%% Check Host.numa_affinity_policy
is_Host.numa_affinity_policy_set?
%% If Host.numa_affinity_policy is "best_effort":
-- Host.numa_affinity_policy is<p><tt>best_effort -->
%% If has_hard_affinity is set, skip numa_placement:
has_hard_affinity?
--"yes"-->exec_xenguest
%% If has_hard_affinity is not set, run numa_placement:
has_hard_affinity?
--"no"-->numa_placement-->exec_xenguest
%% If Host.numa_affinity_policy is off (default, for now),
%% skip NUMA placement:
is_Host.numa_affinity_policy_set?
=="default: disabled"==>
exec_xenguest
end
%% xenguest subgraph
subgraph xenguest
exec_xenguest
==> stub_xc_hvm_build("<tt>stub_xc_hvm_build()")
==> configure_vcpus("<tT>configure_vcpus()")
%% Decision
==> set_hard_affinity?{"
Is <tt>platform/<br>vcpu/#domid/affinity</tt>
set?"}
end
%% do_domctl Hypercalls
numa_placement
--Set the NUMA placement using soft-affinity-->
XEN_VCPUAFFINITY_SOFT("<tt>xc_vcpu_setaffinity(SOFT)")
==> do_domctl
set_hard_affinity?
--yes-->
XEN_VCPUAFFINITY_HARD("<tt>xc_vcpu_setaffinity(HARD)")
--> do_domctl
xc_domain_node_setaffinity("<tt>xc_domain_node_setaffinity()</tt>
and
<tt>xc_domain_node_getaffinity()")
<--> do_domctl
%% Xen subgraph
subgraph xen[Xen Hypervisor]
subgraph domain_update_node_affinity["domain_update_node_affinity()"]
domain_update_node_aff("<tt>domain_update_node_aff()")
==> check_auto_node{"Is domain's<br><tt>auto_node_affinity</tt><br>enabled?"}
=="yes (default)"==>set_node_affinity_from_vcpu_affinities("
Calculate the domain's <tt>node_affinity</tt> mask from vCPU affinity
(used for further NUMA memory allocation for the domain)")
end
do_domctl{"do_domctl()<br>op->cmd=?"}
==XEN_DOMCTL_setvcpuaffinity==>
vcpu_set_affinity("<tt>vcpu_set_affinity()</tt><br>set the vCPU affinity")
==>domain_update_node_aff
do_domctl
--XEN_DOMCTL_setnodeaffinity (not used currently)
-->is_new_affinity_all_nodes?
subgraph domain_set_node_affinity["domain_set_node_affinity()"]
is_new_affinity_all_nodes?{new_affinity<br>is #34;all#34;?}
--is #34;all#34;
--> enable_auto_node_affinity("<tt>auto_node_affinity=1")
--> domain_update_node_aff
is_new_affinity_all_nodes?
--not #34;all#34;
--> disable_auto_node_affinity("<tt>auto_node_affinity=0")
--> domain_update_node_aff
end
%% setting and getting the struct domain's node_affinity:
disable_auto_node_affinity
--node_affinity=new_affinity-->
domain_node_affinity
set_node_affinity_from_vcpu_affinities
==> domain_node_affinity@{ shape: bow-rect,label: "domain: node_affinity" }
--XEN_DOMCTL_getnodeaffinity--> do_domctl
end
click is_Host.numa_affinity_policy_set?
"https://github.com/xapi-project/xen-api/blob/90ef043c1f3a3bc20f1c5d3ccaaf6affadc07983/ocaml/xenopsd/xc/domain.ml#L951-L962"
click numa_placement
"https://github.com/xapi-project/xen-api/blob/90ef043c/ocaml/xenopsd/xc/domain.ml#L862-L897"
click stub_xc_hvm_build
"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L2329-L2436" _blank
click get_flags
"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1164-L1288" _blank
click do_domctl
"https://github.com/xen-project/xen/blob/7cf163879/xen/common/domctl.c#L282-L894" _blank
click domain_set_node_affinity
"https://github.com/xen-project/xen/blob/7cf163879/xen/common/domain.c#L943-L970" _blank
click configure_vcpus
"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1297-L1348" _blank
click set_hard_affinity?
"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1305-L1326" _blank
click xc_vcpu_setaffinity
"https://github.com/xen-project/xen/blob/7cf16387/tools/libs/ctrl/xc_domain.c#L199-L250" _blank
click vcpu_set_affinity
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1353-L1393" _blank
click domain_update_node_aff
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1809-L1876" _blank
click check_auto_node
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1840-L1870" _blank
click set_node_affinity_from_vcpu_affinities
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1867-L1869" _blankxc_domain_node_setaffinity can be used to set the domain’s node_affinity
(which is normally set by xc_set_vcpu_affinity) to different NUMA nodes.
Currently, the node affinity does not affect the Xen scheudler:
In case d->node_affinity would be set before vCPU creation, the initial pCPU
of the new vCPU is the first pCPU of the first NUMA node in the domain’s
node_affinity. This is further changed when one of more cpupools are set up.
As this is only the initial pCPU of the vCPU, this alone does not change the
scheduling of Xen Credit scheduler as it reschedules the vCPUs to other pCPUs.
When done early, before vCPU creation, some domain-related data structures
could be allocated using the domain’s d->node_affinity NUMA node mask.
With further changes in Xen and xenopsd, Xen could allocate the vCPU structs
on the affine NUMA nodes of the domain.
For this, would be that xenopsd would have to call xc_domain_node_setaffinity()
before vCPU creation, after having decided the domain’s NUMA placement,
preferably including claiming the required memory for the domain to ensure
that the domain will be populated from the same NUMA node(s).
This call cannot influence the past: The xenopsd
VM_create
micro-ops calls Xenctrl.domain_create. It currently creates
the domain’s data structures before numa_placement was done.
Improving Xenctrl.domain_create to pass a NUMA node
for allocating the Hypervisor’s data structures (e.g. vCPU)
of the domain would require changes
to the Xen hypervisor and the xenopsd
xenopsd VM_create
micro-op.
xenguest uses
xc_domain_populate_physmap() to populate a Xen domain’s physical memory map:
Both call the XENMEM_populate_physmap hypercall.
xc_domain_populate_physmap_exact() also sets the “exact” flag
for allocating memory only on the given NUMA node.
This is a very simplified overview of the hypercall:
flowchart LR
subgraph hypercall handlers
populate_physmap("<tt>populate_physmap()</tt>
One call for each memory
range (extent)")
end
subgraph "Xen buddy allocator:"
populate_physmap
--> alloc_domheap_pages("<tt>alloc_domheap_pages()</tt>
Assign allocated pages to
the domain")
alloc_domheap_pages
--> alloc_heap_pages("<tt>alloc_heap_pages()</tt>
If needed: split high-order
pages into smaller buddies,
and scrub dirty pages")
--> get_free_buddy("<tt>get_free_buddy()</tt>
If reqested: Allocate from a
preferred/exact NUMA node
and/or from
unscrubbed memory
")
end
click populate_physmap
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L159-L314
" _blank
click alloc_domheap_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L2641-L2697
" _blank
click get_free_buddy
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L855-L958
" _blank
click alloc_heap_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L967-L1116
" _blankIt calls
construct_memop_from_reservation()
to convert the arguments for allocating a page from
struct xen_memory_reservation
to struct memop_args and
passes
it to
populate_physmap():
It populates struct memop_args using the
hypercall arguments. It:
extent_start, nr_extents, and extent_order.memop_args->memflags using the given mem_flags.When a vNUMA vnode is passed using XENMEMF_vnode, and domain->vnuma
and domain->vnuma->nr_vnodes are set, and the
vnode (virtual NUMA node) maps to a pnode (physical NUMA node), it also:
pnode in the memflags of the struct memop_argsXENMEMF_exact_node_request in them as well.If no vNUMA node is passed, construct_memop_from_reservation()
calls
propagate_node()
to propagate the NUMA node and XENMEMF_exact_node_request for use in Xen.
It handles hypercall preemption and resumption after a preemption, keeps track of the already populated pages.
For each range (extent), it runs an iteration of the allocation loop. It passes the
struct domainmemflagsIt calls alloc_heap_pages() and on success, assigns the allocated pages to the domain.
It calls get_free_buddy()[^1] to allocate a page at the most suitable place: When no pages of the requested size are free, it splits larger superpages into pages of the requested size.
It finds memory based on the flags and domain and return its page struct:
For details see the get_free_buddy() finds memory based on the flags and domain.
flowchart TD
subgraph XenCtrl
xc_domain_populate_physmap["<tt>xc_domain_populate_physmap()"]
xc_domain_populate_physmap_exact["<tt>xc_domain_populate_physmap_exact()"]
end
subgraph Xen
%% sub-subgraph from memory_op() to populate_node() and back
xc_domain_populate_physmap & xc_domain_populate_physmap_exact
<--reservation,<br>and for preempt:<br>nr_start/nr_done-->
memory_op("<tt>memory_op(XENMEM_populate_physmap)")
memory_op
--struct xen_memory_reservation-->
construct_memop_from_reservation("<tt>construct_memop_from_reservation()")
--struct<br>xen_memory_reservation->mem_flags-->
propagate_node("<tt>propagate_node()")
--_struct<br>memop_args->memflags_-->
construct_memop_from_reservation
--_struct memop_args_-->
memory_op<--struct memop_args *:
struct domain *,
List of extent base addrs,
Number of extents,
Size of each extent (extent_order),
Allocation flags(memflags)-->
populate_physmap[["<tt>populate_physmap()"]]
<-.domain, extent base addrs, extent size, memflags, nr_start and nr_done.->
populate_physmap_loop--if memflags & MEMF_populate_on_demand -->guest_physmap_mark_populate_on_demand("
<tt>guest_physmap_mark_populate_on_demand()")
populate_physmap_loop@{ label: "While extents to populate,
and not asked to preempt,
for each extent left to do:", shape: notch-pent }
--domain, order, memflags-->
alloc_domheap_pages("<tt>alloc_domheap_pages()")
--zone_lo, zone_hi, order, memflags, domain-->
alloc_heap_pages
--zone_lo, zone_hi, order, memflags, domain-->
get_free_buddy("<tt>get_free_buddy()")
--_page_info_
-->alloc_heap_pages
--if no page-->
no_scrub("<tt>get_free_buddy(MEMF_no_scrub)</tt>
(honored only when order==0)")
--_dirty 4k page_
-->alloc_heap_pages
<--_dirty 4k page_-->
scrub_one_page("<tt>scrub_one_page()")
alloc_heap_pages("<tt>alloc_heap_pages()</tt>
(also splits higher-order pages
into smaller buddies if needed)")
--_page_info_
-->alloc_domheap_pages
--page_info, order, domain, memflags-->assign_page("<tt>assign_page()")
assign_page
--page_info, nr_mfns, domain, memflags-->
assign_pages("<tt>assign_pages()")
--domain, nr_mfns-->
domain_adjust_tot_pages("<tt>domain_adjust_tot_pages()")
alloc_domheap_pages
--_page_info_-->
populate_physmap_loop
--page(gpfn, mfn, extent_order)-->
guest_physmap_add_page("<tt>guest_physmap_add_page()")
populate_physmap--nr_done, preempted-->memory_op
end
click memory_op
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L1409-L1425
" _blank
click construct_memop_from_reservation
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L1022-L1071
" _blank
click propagate_node
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L524-L547
" _blank
click populate_physmap
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L159-L314
" _blank
click populate_physmap_loop
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L197-L304
" _blank
click guest_physmap_mark_populate_on_demand
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L210-220
" _blank
click guest_physmap_add_page
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L296
" _blank
click alloc_domheap_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L2641-L2697
" _blank
click alloc_heap_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L967-L1116
" _blank
click get_free_buddy
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L855-L958
" _blank
click assign_page
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L2540-L2633
" _blank
click assign_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L2635-L2639
" _blankIn the Xen hypervisor, each vCPU has:
A soft affinity, This is the list of pCPUs where a vCPU prefers to run:
This can be used in cases to make vCPUs prefer to run on a set on pCPUs, for example the pCPUs of a NUMA node, but in case those are already busy, the Credit schedule can still ignore the soft-affinity. A typical use case for this are NUMA machines, where the soft affinity for the vCPUs of a domain should be set equal to the pCPUs of the NUMA node where the domain’s memory shall be placed.
See the description of the NUMA feature for more details.
A hard affinity, also known as pinning. This is the list of pCPUs where a vCPU is allowed to run
Hard affinity is currently not used for NUMA placement, but can be configured
manually for a given domain, either using xe VCPUs-params:mask= or the API.
For example, the vCPU’s pinning can be configured for a VM with:1
xe vm-param-set uuid=<template_uuid> vCPUs-params:mask=1,2,3There are also host-level guest_VCPUs_params which are used by
host-cpu-tune to exclusively pin Dom0 and guests (i.e. that their
pCPUs never overlap). Note: This isn’t currently supported by the
NUMA code: It could result that the NUMA placement picks a node that
has reduced capacity or unavailable due to the host mask that
host-cpu-tune has set.
The libxenctrl library call xc_set_vcpu_affinity()
controls the pCPU affinity of the given vCPU.
xenguest
uses it when building domains if
xenopsd
added vCPU affinity information to the XenStore platform data path
platform/vcpu/#domid/affinity of the domain.
Besides that, xc_set_vcpu_affinity() can also modify the NUMA node
affinity of the Xen domain if the vCPU:
When Xen creates a domain, it enables the domain’s d->auto_node_affinity
feature flag.
When it is enabled, setting the vCPU affinity also updates the NUMA node affinity which is used for memory allocations for the domain:
flowchart TD
subgraph libxenctrl
xc_vcpu_setaffinity("<tt>xc_vcpu_setaffinity()")--hypercall-->xen
end
subgraph xen[Xen Hypervisor]
direction LR
vcpu_set_affinity("<tt>vcpu_set_affinity()</tt><br>set the vCPU affinity")
-->check_auto_node{"Is the domain's<br><tt>auto_node_affinity</tt><br>enabled?"}
--"yes<br>(default)"-->
auto_node_affinity("Set the<br>domain's<br><tt>node_affinity</tt>
mask as well<br>(used for further<br>NUMA memory<br>allocation)")
click xc_vcpu_setaffinity
"https://github.com/xen-project/xen/blob/7cf16387/tools/libs/ctrl/xc_domain.c#L199-L250" _blank
click vcpu_set_affinity
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1353-L1393" _blank
click domain_update_node_aff
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1809-L1876" _blank
click check_auto_node
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1840-L1870" _blank
click auto_node_affinity
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1867-L1869" _blank
endWhen Host.numa_affinity_policy is set to
best_effort,
xenopsd attempts NUMA node placement
when building new VMs and instructs
xenguest
to set the vCPU affinity of the domain.
With the domain’s auto_node_affinity flag enabled by default in Xen,
this automatically also sets the d->node_affinity mask of the domain.
This then causes the Xen memory allocator to prefer the NUMA nodes in the
d->node_affinity NUMA node mask when allocating memory.
That is, (for completeness) unless Xen’s allocation function
alloc_heap_pages() receives a specific NUMA node in its memflags
argument when called.
See xc_domain_node_setaffinity() for more
information about another way to set the node_affinity NUMA node mask
of Xen domains and more depth on how it is used in Xen.
In the flowchart, two code paths are set in bold:
Host.numa_affinity_policy is the default (off) in xenopsd.xc_vcpu_setaffinity(XEN_VCPUAFFINITY_SOFT) in Xen,
when the Domain’s auto_node_affinity flag is enabled (default) to show
how it changes to the vCPU affinity update the domain’s node_affinity
in this default case as well.xenguest uses the Xenstore to read the static domain configuration that it needs reads to build the domain.
flowchart TD
subgraph VM.create["xenopsd VM.create"]
%% Is xe vCPU-params:mask= set? If yes, write to Xenstore:
is_xe_vCPUparams_mask_set?{"
Is
<tt>xe vCPU-params:mask=</tt>
set? Example: <tt>1,2,3</tt>
(Is used to enable vCPU<br>hard-affinity)
"} --"yes"--> set_hard_affinity("Write hard-affinity to XenStore:
<tt>platform/vcpu/#domid/affinity</tt>
(xenguest will read this and other configuration data
from Xenstore)")
end
subgraph VM.build["xenopsd VM.build"]
%% Labels of the decision nodes
is_Host.numa_affinity_policy_set?{
Is<p><tt>Host.numa_affinity_policy</tt><p>set?}
has_hard_affinity?{
Is hard-affinity configured in <p><tt>platform/vcpu/#domid/affinity</tt>?}
%% Connections from VM.create:
set_hard_affinity --> is_Host.numa_affinity_policy_set?
is_xe_vCPUparams_mask_set? == "no"==> is_Host.numa_affinity_policy_set?
%% The Subgraph itself:
%% Check Host.numa_affinity_policy
is_Host.numa_affinity_policy_set?
%% If Host.numa_affinity_policy is "best_effort":
-- Host.numa_affinity_policy is<p><tt>best_effort -->
%% If has_hard_affinity is set, skip numa_placement:
has_hard_affinity?
--"yes"-->exec_xenguest
%% If has_hard_affinity is not set, run numa_placement:
has_hard_affinity?
--"no"-->numa_placement-->exec_xenguest
%% If Host.numa_affinity_policy is off (default, for now),
%% skip NUMA placement:
is_Host.numa_affinity_policy_set?
=="default: disabled"==>
exec_xenguest
end
%% xenguest subgraph
subgraph xenguest
exec_xenguest
==> stub_xc_hvm_build("<tt>stub_xc_hvm_build()")
==> configure_vcpus("<tT>configure_vcpus()")
%% Decision
==> set_hard_affinity?{"
Is <tt>platform/<br>vcpu/#domid/affinity</tt>
set?"}
end
%% do_domctl Hypercalls
numa_placement
--Set the NUMA placement using soft-affinity-->
XEN_VCPUAFFINITY_SOFT("<tt>xc_vcpu_setaffinity(SOFT)")
==> do_domctl
set_hard_affinity?
--yes-->
XEN_VCPUAFFINITY_HARD("<tt>xc_vcpu_setaffinity(HARD)")
--> do_domctl
xc_domain_node_setaffinity("<tt>xc_domain_node_setaffinity()</tt>
and
<tt>xc_domain_node_getaffinity()")
<--> do_domctl
%% Xen subgraph
subgraph xen[Xen Hypervisor]
subgraph domain_update_node_affinity["domain_update_node_affinity()"]
domain_update_node_aff("<tt>domain_update_node_aff()")
==> check_auto_node{"Is domain's<br><tt>auto_node_affinity</tt><br>enabled?"}
=="yes (default)"==>set_node_affinity_from_vcpu_affinities("
Calculate the domain's <tt>node_affinity</tt> mask from vCPU affinity
(used for further NUMA memory allocation for the domain)")
end
do_domctl{"do_domctl()<br>op->cmd=?"}
==XEN_DOMCTL_setvcpuaffinity==>
vcpu_set_affinity("<tt>vcpu_set_affinity()</tt><br>set the vCPU affinity")
==>domain_update_node_aff
do_domctl
--XEN_DOMCTL_setnodeaffinity (not used currently)
-->is_new_affinity_all_nodes?
subgraph domain_set_node_affinity["domain_set_node_affinity()"]
is_new_affinity_all_nodes?{new_affinity<br>is #34;all#34;?}
--is #34;all#34;
--> enable_auto_node_affinity("<tt>auto_node_affinity=1")
--> domain_update_node_aff
is_new_affinity_all_nodes?
--not #34;all#34;
--> disable_auto_node_affinity("<tt>auto_node_affinity=0")
--> domain_update_node_aff
end
%% setting and getting the struct domain's node_affinity:
disable_auto_node_affinity
--node_affinity=new_affinity-->
domain_node_affinity
set_node_affinity_from_vcpu_affinities
==> domain_node_affinity@{ shape: bow-rect,label: "domain: node_affinity" }
--XEN_DOMCTL_getnodeaffinity--> do_domctl
end
click is_Host.numa_affinity_policy_set?
"https://github.com/xapi-project/xen-api/blob/90ef043c1f3a3bc20f1c5d3ccaaf6affadc07983/ocaml/xenopsd/xc/domain.ml#L951-L962"
click numa_placement
"https://github.com/xapi-project/xen-api/blob/90ef043c/ocaml/xenopsd/xc/domain.ml#L862-L897"
click stub_xc_hvm_build
"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L2329-L2436" _blank
click get_flags
"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1164-L1288" _blank
click do_domctl
"https://github.com/xen-project/xen/blob/7cf163879/xen/common/domctl.c#L282-L894" _blank
click domain_set_node_affinity
"https://github.com/xen-project/xen/blob/7cf163879/xen/common/domain.c#L943-L970" _blank
click configure_vcpus
"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1297-L1348" _blank
click set_hard_affinity?
"https://github.com/xenserver/xen.pg/blob/65c0438b/patches/xenguest.patch#L1305-L1326" _blank
click xc_vcpu_setaffinity
"https://github.com/xen-project/xen/blob/7cf16387/tools/libs/ctrl/xc_domain.c#L199-L250" _blank
click vcpu_set_affinity
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1353-L1393" _blank
click domain_update_node_aff
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1809-L1876" _blank
click check_auto_node
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1840-L1870" _blank
click set_node_affinity_from_vcpu_affinities
"https://github.com/xen-project/xen/blob/7cf16387/xen/common/sched/core.c#L1867-L1869" _blankThe VM parameter VCPUs-params:mask is documented in the official XAPI user documentation. ↩︎