linux-kernel - Re: [PATCH printk v2 25/26] rcu: Mark emergency section in rcu stalls

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Date: Fri, 1 Mar 2024 16:13:27 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <quic_neeraju@...cinc.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu@...r.kernel.org
Subject: Re: [PATCH printk v2 25/26] rcu: Mark emergency section in rcu stalls

On Sun 2024-02-18 20:03:25, John Ogness wrote:
> Mark an emergency section within print_other_cpu_stall(), where
> RCU stall information is printed. In this section, the CPU will
> not perform console output for the printk() calls. Instead, a
> flushing of the console output is triggered when exiting the
> emergency section.
>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Reviewed-by: Petr Mladek <pmladek@...e.com>

I was just curious about one thing. But it seems to work well.

print_other_cpu_stall() print backtraces on other CPUs via NMI.
The other CPUs would not see the emergency context. They would
call defer_console_output() because they are in NMI. As a result:

  + Legacy consoles might be flushed on other CPUs even before
    nbcon_cpu_emergency_exit() gets called.

  + nbcon consoles might still be flushed by the printk kthread
    until all messages get flushed directly by nbcon_cpu_emergency_exit()

As I wrote. The behavior is corrent. It was just not obvious to me.


> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -9,6 +9,7 @@
>  
>  #include <linux/kvm_para.h>
>  #include <linux/rcu_notifier.h>
> +#include <linux/console.h>
>  
>  //////////////////////////////////////////////////////////////////////////////
>  //
> @@ -604,6 +605,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
>  	if (rcu_stall_is_suppressed())
>  		return;
>  
> +	nbcon_cpu_emergency_enter();
> +
>  	/*
>  	 * OK, time to rat on our buddy...
>  	 * See Documentation/RCU/stallwarn.rst for info on how to debug
> @@ -658,6 +661,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
>  	panic_on_rcu_stall();
>  
>  	rcu_force_quiescent_state();  /* Kick them all. */
> +
> +	nbcon_cpu_emergency_exit();
>  }
>  
>  static void print_cpu_stall(unsigned long gps)

Best Regards,
Petr