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Message-ID: <d8513908-3348-2c81-9f82-a6314a34bb74@squashfs.org.uk>
Date:   Thu, 24 Nov 2022 19:32:02 +0000
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Subject: Re: BUG: BISECTED: in squashfs_xz_uncompress() (Was: RCU stalls in
 squashfs_readahead())
To:     Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>,
        paulmck@kernel.org
Cc:     "Elliott, Robert (Servers)" <elliott@hpe.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "phillip.lougher@gmail.com" <phillip.lougher@gmail.com>,
        Thorsten Leemhuis <regressions@leemhuis.info>
References: <20221120175016.GC4001@paulmck-ThinkPad-P17-Gen-1>
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 <9486bc0a-345e-a93c-581f-50e2a06a3314@alu.unizg.hr>
From:   Phillip Lougher <phillip@squashfs.org.uk>
In-Reply-To: <9486bc0a-345e-a93c-581f-50e2a06a3314@alu.unizg.hr>
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On 24/11/2022 18:04, Mirsad Goran Todorovac wrote:
> On 24. 11. 2022. 18:19, Paul E. McKenney wrote:
>> On Thu, Nov 24, 2022 at 06:06:13PM +0100, Mirsad Goran Todorovac wrote:
>>> On 23. 11. 2022. 20:09, Paul E. McKenney wrote:
>>>
>>>>>> If you build with (say) CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=200, does
>>>>>> this still happen?
>>>>>
>>>>> BTW, you don't need to rebuild the kernel to change those 
>>>>> parameters; they're
>>>>> module parameters, so can be modified on the kernel command line 
>>>>> (if needed
>>>>> during boot) and sysfs (if only needed after boot).
>>>>>
>>>>> For sysfs the syntax is:
>>>>> #!/bin/bash
>>>>> # set rcu timeouts to specified values
>>>>> echo 60 > /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout
>>>>> echo 21000 > /sys/module/rcupdate/parameters/rcu_exp_cpu_stall_timeout
>>>>> echo 600000 > /sys/module/rcupdate/parameters/rcu_task_stall_timeout
>>>>> grep -Hs . /sys/module/rcupdate/parameters/rcu_*_timeout
>>>>
>>>> Excellent point, thank you!
>>>>
>>>> I hope that this makes Mirsad's life easier, perhaps featuring less 
>>>> time
>>>> waiting for kernel builds and reboots.  ;-)
>>>
>>> Unfortunately, the first stall and NMI occurs before any system 
>>> script or setting a /sys/module/rcupdate/parameters/*
>>> could be executed, as second 14 of the boot process:
>>>
>>> [   14.320045] rcu: INFO: rcu_preempt detected expedited stalls on 
>>> CPUs/tasks: { 7-.... } 6 jiffies s: 105 root: 0x80/.
>>> [   14.320064] rcu: blocking rcu_node structures (internal RCU debug):
> 
> ...
> 
>>> Probably something sensible should be set in the case of KASAN build. 
>>> This example of stall
>>> apparently has nothing to do with squashfs_readahead().
>>
>> Can't have everything, I guess!
>>
>> How about building your kernel with CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=200?
>> Again, mainline defaults to 21000.
> 
> Did just that, and so far there is no modprobe stall in second 14 of 
> boot at least. Looks good.
> Probably it is too early to say anything in general before more uptime 
> and stress load.
> 
> BTW, the 20 for CONFIG_RCU_EXP_STALL_TIMEOUT wasn't my invention, but it 
> comes from generic
> Ubuntu stock kernel (but without KASAN or KMEMLEAK config options):
> 
> # grep STALL /boot/config-5.19.5-051905-generic
> CONFIG_RCU_STALL_COMMON=y
> # CONFIG_HEADERS_INSTALL is not set
> CONFIG_RCU_CPU_STALL_TIMEOUT=60
> CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=20 > #

That has been raised as a bug, and a fix has been committed.

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1991951


> 
> Thanks,
> Mirsad
> 
> -- 
> Mirsad Goran Todorovac
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