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@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
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
