PV-vNUMA issue: topology is misinterpreted by the guest

[Dario Faggioli <dario.faggioli@citrix.com>]

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Hey,

This started on IRC, but it's actually appropriate to have the
conversation here.

I just discovered an issue with vNUMA, when PV guests are used. In fact,
creating a 4 vCPUs PV guest, and making up things so that all the 4
vCPUs should be busy, I see this:

root@Zhaman:~# xl vcpu-list test
Name                                ID  VCPU   CPU State   Time(s) Affinity (Hard / Soft)
test                                 4     0    5   r--    1481.9  all / 0-7
test                                 4     1    2   r--    1479.4  all / 0-7
test                                 4     2   15   -b-       7.5  all / 8-15
test                                 4     3   10   -b-    1324.8  all / 8-15

Going checking inside the guest, confirms that *everything* runs on
vCPUs 0 and 1. However, using schedtool or taskset, I can force tasks to
execute on vCPUs 2 and 3.

Inspecting the guest's dmesg, I've seen this:

[    0.128416] ------------[ cut here ]------------
[    0.128416] WARNING: CPU: 2 PID: 0 at ../arch/x86/kernel/smpboot.c:317 topology_sane.isra.2+0x74/0x88()
[    0.128416] sched: CPU #2's smt-sibling CPU #0 is not on the same node! [node: 1 != 0]. Ignoring dependency.
[    0.128416] Modules linked in:
[    0.128416] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 3.19.0+ #1
[    0.128416]  0000000000000009 ffff88001ee3bdd0 ffffffff81657c7b ffffffff810bbd2c
[    0.128416]  ffff88001ee3be20 ffff88001ee3be10 ffffffff81081510 ffff88001ee3bea0
[    0.128416]  ffffffff8103aa02 ffff88003ea0a001 0000000000000000 ffff88001f20a040
[    0.128416] Call Trace:
[    0.128416]  [<ffffffff81657c7b>] dump_stack+0x4f/0x7b
[    0.128416]  [<ffffffff810bbd2c>] ? up+0x39/0x3e
[    0.128416]  [<ffffffff81081510>] warn_slowpath_common+0xa1/0xbb
[    0.128416]  [<ffffffff8103aa02>] ? topology_sane.isra.2+0x74/0x88
[    0.128416]  [<ffffffff81081570>] warn_slowpath_fmt+0x46/0x48
[    0.128416]  [<ffffffff8101eeb1>] ? __cpuid.constprop.0+0x15/0x19
[    0.128416]  [<ffffffff8103aa02>] topology_sane.isra.2+0x74/0x88
[    0.128416]  [<ffffffff8103ac70>] set_cpu_sibling_map+0x21a/0x444
[    0.128416]  [<ffffffff81056ac3>] ? numa_add_cpu+0x98/0x9f
[    0.128416]  [<ffffffff8100b8f2>] cpu_bringup+0x63/0xa8
[    0.128416]  [<ffffffff8100b945>] cpu_bringup_and_idle+0xe/0x1a
[    0.128416] ---[ end trace 95bff1aef57ee1b1 ]---

So, basically, Linux is complaining that we're trying to put two vCPUs,
that looks to be SMT siblings, on different NUMA nodes. And, yes, I
think this is quite disruptive for the Linux's scheduler internal logic.

The vnuma bits of the guest config are these:

 vnuma = [ [ "pnode=0","size=512","vcpus=0-1","vdistances=10,20"  ],
           [ "pnode=1","size=512","vcpus=2-3","vdistances=20,10"  ] ]

From inside the guest, the topology looks to be like this:

root@test:~# numactl --hardware
available: 2 nodes (0-1)
node 0 cpus: 0 1
node 0 size: 475 MB
node 0 free: 382 MB
node 1 cpus: 2 3
node 1 size: 495 MB
node 1 free: 475 MB
node distances:
node   0   1 
  0:  10  10 
  1:  20  10

root@test:~# cat /sys/devices/system/cpu/cpu0/topology/thread_siblings_list 
0-1
root@test:~# cat /sys/devices/system/cpu/cpu0/topology/core_siblings_list 
0-3
root@test:~# cat /sys/devices/system/cpu/cpu2/topology/thread_siblings_list 
2-3
root@test:~# cat /sys/devices/system/cpu/cpu2/topology/core_siblings_list 
0-3

So the complain during boot seems to be against 'core_siblings' (which
was not what I expected, but perhaps I misremember the meaning of
"core_siblings" VS. "thread_siblings" VS. smt-siblings in Linux; I'll
double check).

Anyway, is there anything we can do to fix or workaround things?

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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