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Date:   Thu, 24 Nov 2022 12:15:52 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Cc:     "Elliott, Robert (Servers)" <elliott@hpe.com>,
        Phillip Lougher <phillip@squashfs.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        "phillip.lougher@gmail.com" <phillip.lougher@gmail.com>,
        Thorsten Leemhuis <regressions@leemhuis.info>
Subject: Re: BUG: BISECTED: in squashfs_xz_uncompress() (Was: RCU stalls in
 squashfs_readahead())
Message-ID: <20221124201552.GV4001@paulmck-ThinkPad-P17-Gen-1>
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On Thu, Nov 24, 2022 at 07:04:13PM +0100, 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
> #

Gotta admit, those Ubuntu folks do not lack for bravery!!!  Even if they
don't also enable KASAN or KMEMLEAK by default.  ;-)

							Thanx, Paul