Has anyone seen this recently? I couldn't replicate it on:
- CentOS 6 running kernel-2.6.32-696.16.1.el6.x86_64, kernel-lt-4.4.105-1.el6.elrepo.x86_64
- CentOS 7 running 4.9.67-1.el7.centos.x86_64

But I can replicate it consistently running "xl -v vcpu-set <domU> <val>" on:
- CentOS 6 running 4.14.5-1.el6.elrepo.x86_64
- CentOS 7 running 4.14.5-1.el7.elrepo.x86_64

dom0 versions tested with similar results in the domU:
- 4.6.6-6.el7 on kernel 4.9.63-29.el7.x86_64
- 4.6.3-15.el6 on kernel 4.9.37-29.el6.x86_64

Noticed behaviour:
- These commands stall:
top
ls -l /var/tmp
ls -l /tmp
- Stuck in D state on the CentOS 7 domU:
root         5  0.0  0.0      0     0 ?        D    11:20   0:00 [kworker/u8:0]
root       316  0.0  0.0      0     0 ?        D    11:20   0:00 [jbd2/xvda1-8]
root      1145  0.0  0.2 116636  4776 ?        Ds   11:20   0:00 -bash
root      1289  0.0  0.1  25852  2420 ?        Ds   11:35   0:00 /usr/bin/systemd-tmpfiles --clean
root      1290  0.0  0.1 125248  2696 pts/1    D+   11:44   0:00 ls --color=auto -l /tmp/
root      1293  0.0  0.1 125248  2568 pts/2    D+   11:44   0:00 ls --color=auto -l /var/tmp
root      1296  0.0  0.2 116636  4908 pts/3    Ds+  11:44   0:00 -bash
root      1358  0.0  0.1 125248  2612 pts/4    D+   11:47   0:00 ls --color=auto -l /var/tmp

At a first glance it appears the issue is in 4.14.5 kernel. Stack traces follow:

Adi Pircalabu