[Bug 253087] Timeout of bufdaemon happens at shutdown time with -CURRENT amd64 and VirtulaBox VM

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253087

            Bug ID: 253087
           Summary: Timeout of bufdaemon happens at shutdown time with
                    -CURRENT amd64 and VirtulaBox VM
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: yasu at utahime.org
                CC: kib at FreeBSD.org

[Host]
CPU: Intel Core i7 9700 3.00GHz
OS: 64bit Windows 10 20H2
VirtualBox: 6.1.18

[VM]
CPU: 4 core
Mem: 8GB
HDD: 100GB

Since December I've been experiencing the problem that timeout of bufdaemon
happens at shutdown time with -CURRENT amd64 and VirtualBox VM under above
conditions. The problem happens when I login the VM and do something to certain
extent. For example the problem is reproducible by doing `make buildworld`.

The same problem was reported by AMD Ryzen users at freebsd-current ML and fix
for the CPU was already committed. But in my case CPU is Intel and fix for AMD
Ryzen doesn't solve the problem of course. So I tried bisect of source tree and
found that following commit is the source of the problem.

----------------------------------------------------------------------
commit 84eaf2ccc6aa05da7b7389991d3023698b756e3f
Author: Konstantin Belousov <kib at FreeBSD.org>
Date:   Mon Dec 21 19:02:31 2020 +0200

    x86: stop punishing VMs with low priority for TSC timecounter

    I suspect that virtualization techniques improved from the time when we
    have to effectively disable TSC use in VM.  For instance, it was reported
    (complained) in https://github.com/JuliaLang/julia/issues/38877 that
    FreeBSD is groundlessly slow on AWS with some loads.

    Remove the check and start watching for complaints.

    Reviewed by:    emaste, grehan
    Discussed with: cperciva
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D27629
----------------------------------------------------------------------

I confirmed that the problem still happens with
f17fc5439f517d06ac8976f53354789cde5a7427 committed at Fri Jan 29 09:34:27 2021
-0500 but reverting above one fixes the problem. Moreover same problem happens
with 40cb0344eb27e0bb9a112ff50812a7e77816d6be of stable/13 committed at Thu Jan
28 18:59:57 2021 -0500 and and reverting above one also fixes the problem.

Cc-ing the committer of 84eaf2ccc6aa05da7b7389991d3023698b756e3f

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