Re: ppc64le: `NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #20!!!` when turning off SMT
From: Frederic Weisbecker <frederic@kernel.org>
Date: 2022-02-08 13:23:21
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On Tue, Feb 08, 2022 at 08:32:37AM +0100, Paul Menzel wrote:
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).I see, so I assume this sets some CPUs offline, right?
$ dmesg [ 0.000000] Linux version 5.17.0-rc1+ (pmenzel@flughafenberlinbrandenburgwillybrandt.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!!!
That's IRQ_POLL_SOFTIRQ. The problem here is probably that some of these softirqs are pending even though ksoftirqd has been parked. I see there is irq_poll_cpu_dead() that migrates the pending queue once the CPU is finally dead, so this is well handled. I'm preparing a patch to fix the warning. Thanks.