Re: [PATCH] docs: Document panic_on_rcu_stall default behavior

[Kunwu Chan]

May 11, 2026 at 3:54 AM, "Paul E. McKenney" <paulmck@xxxxxxxxxx mailto:paulmck@xxxxxxxxxx?to=%22Paul%20E.%20McKenney%22%20%3Cpaulmck%40kernel.org%3E > wrote:


> 
> On Sat, May 09, 2026 at 05:12:14PM +0800, Kunwu Chan wrote:
> 
> > 
> > From: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> >  
> >  Commit ab875b3e179f ("rcu: Add BOOTPARAM_RCU_STALL_PANIC
> >  Kconfig option") made the default value of
> >  kernel.panic_on_rcu_stall depend on
> >  CONFIG_BOOTPARAM_RCU_STALL_PANIC.
> >  
> >  Document this in kernel.rst
> >  
> >  Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> > 
> This commit depends on the commit you call out above, which, given Linus
> Torvalds's reaction, is unlikely to make it into mainline. :-(
> 
> A likely workaround is to use the existing sysctl kernel boot parameter,
> as in: "sysctl.kernel.panic_on_rcu_stall=1".
> 
> This can also be embedded into the kernel image using the bootconfig
> facility. To do this, build your kernel with the following Kconfig
> options:
> 
>  CONFIG_BOOT_CONFIG=y
>  CONFIG_BOOT_CONFIG_FORCE=y
>  CONFIG_BOOT_CONFIG_EMBED=y
>  CONFIG_BOOT_CONFIG_EMBED_FILE=".bootconfig"
> 
Hi Paul,

Thank you for the detailed explanation and the alternative solutions.
I wasn't aware of the feedback from Linus regarding the dependency commit. 
Since it's unlikely to be merged into mainline, :-(
it makes sense to drop this documentation patch as well.

> Then create your ".bootconfig" file in the top-level directory of your
> Linux-kernel source tree:
> 
>  kernel {
>  sysctl.kernel.panic_on_rcu_stall=1
>  }
> 
> You can also pass parameters to the "init" process by adding an "init"
> stanza to your .bootconfig file. See the Linux-kernel bootconfig
> documentation for more information:
> 
>  Documentation/admin-guide/bootconfig.rst
> 
I will look into the bootconfig and sysctl approaches you suggested. 
Thanks again for pointing me in the right direction!

>  Thanx, Paul
> 

Best regards,
Kunwu Chan
> > 
> > ---
> >  Documentation/admin-guide/sysctl/kernel.rst | 4 ++++
> >  1 file changed, 4 insertions(+)
> >  
> >  diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> >  index c6994e55d141..99598a83f830 100644
> >  --- a/Documentation/admin-guide/sysctl/kernel.rst
> >  +++ b/Documentation/admin-guide/sysctl/kernel.rst
> >  @@ -948,6 +948,10 @@ panic_on_rcu_stall
> >  When set to 1, calls panic() after RCU stall detection messages. This
> >  is useful to define the root cause of RCU stalls using a vmcore.
> >  
> >  +The default value can be configured at build time via
> >  +``CONFIG_BOOTPARAM_RCU_STALL_PANIC``. Runtime updates to this sysctl
> >  +always override the built-in default.
> >  +
> >  = ============================================================
> >  0 Do not panic() when RCU stall takes place, default behavior.
> >  1 panic() after printing RCU stall messages.
> >  -- 
> >  2.43.0
> >
>