linux-kernel - ppc64le: `NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #20!!!` when turning off SMT

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Date:   Tue, 8 Feb 2022 08:32:37 +0100
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...ts.ozlabs.org
Subject: ppc64le: `NOHZ tick-stop error: Non-RCU local softirq work is
 pending, handler #20!!!` when turning off SMT

Dear Linux folks,

On the POWER8 server IBM S822LC running Ubuntu 21.10, Linux 5.17-rc1+ 
built with

     $ grep HZ /boot/config-5.17.0-rc1+
     CONFIG_NO_HZ_COMMON=y
     # CONFIG_HZ_PERIODIC is not set
     CONFIG_NO_HZ_IDLE=y
     # CONFIG_NO_HZ_FULL is not set
     CONFIG_NO_HZ=y
     # CONFIG_HZ_100 is not set
     CONFIG_HZ_250=y
     # CONFIG_HZ_300 is not set
     # CONFIG_HZ_1000 is not set
     CONFIG_HZ=250

once warned about a NOHZ tick-stop error, when I executed `sudo 
/usr/sbin/ppc64_cpu --smt=off` (so that KVM would work).

```
$ dmesg
[    0.000000] Linux version 5.17.0-rc1+ 
(pmenzel@...ghafenberlinbrandenburgwillybrandt.molgen.mpg.de) (Ubuntu 
clang version 13.0.0-2, LLD 13.0.0) #1 SMP Fri Jan 28 17:13:04 CET 2022
[…]
[271272.030262] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271272.305726] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271272.549790] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271274.885167] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271275.113896] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271275.412902] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271275.625245] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271275.833107] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271276.041391] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
[271277.244880] NOHZ tick-stop error: Non-RCU local softirq work is 
pending, handler #20!!!
```

Kind regards,

Paul