Re: BUG: BISECTED: in squashfs_xz_uncompress() (Was: RCU stalls in squashfs_readahead())

[Mirsad Goran Todorovac]

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