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Date:   Tue, 6 Dec 2022 21:35:57 +0100
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Subject: Re: BUG: BISECTED: in squashfs_xz_uncompress() (Was: RCU stalls in
 squashfs_readahead())
To:     Phillip Lougher <phillip@squashfs.org.uk>, 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>
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From:   Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
In-Reply-To: <d8513908-3348-2c81-9f82-a6314a34bb74@squashfs.org.uk>
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On 24. 11. 2022. 20:32, Phillip Lougher wrote:
> 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

P.S.

As for the comment that I am thrashing my systems, I now test activated MG-LRU kernel option
with 6.1-rc8 build and it functions much better, with no multimedia lags or chirps, even
with only 130/8192 MiB free and 5/10 GiB in swap area.

I am running basically the same load of simultaneously opened Firefox, Chrome and Thunderbird
windows.

However, I have set CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=0

The conclusion is that the squashfs isn't blocking, but 6 to 8 jiffies were not enough to
complete the operation, so other CPUs issued NMIs. With longer timeout, it is evident that
it was a longer operation due to KASAN build and not a lockup.

So I think I have to apologise to have wasted so much of your time with a false alarm.

To summarise, the culprit was obviously the CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=20 setting
from the Ubuntu mainline kernel stock, which I unsuspectedly copied into my build and
made a recommended "make olddefconfig".

Thanks,
Mirsad

--
Mirsad Goran Todorovac
Sistem inženjer
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu
-- 
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb, Republic of Croatia
The European Union