Re: [PATCH RFC] sysrq,rcu: suppress RCU stall warnings while sysrq runs

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 24, 2014 at 09:04:52AM -0400, Rik van Riel wrote:
> As suggested by Paul McKenney...
> 
> This seems small and simple enough that it should not introduce any
> extra maintenance work in the future.
> 
> ---8<---
> 
> Some sysrq handlers can run for a long time, because they dump a lot
> of data onto a serial console. Having RCU stall warnings pop up in
> the middle of them only makes the problem worse.
> 
> This patch temporarily disables RCU stall warnings while a sysrq
> request is handled.
> 
> Signed-off-by: Rik van Riel <riel@redhat.com>
> Suggested-by: Paul McKenney <paulmck@linux.vnet.ibm.com>

I guess this means you like the idea.  ;-)

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
>  drivers/tty/sysrq.c      |  2 ++
>  include/linux/rcupdate.h |  2 ++
>  kernel/rcu/update.c      | 11 +++++++++++
>  3 files changed, 15 insertions(+)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 3c61e9b..dd53af9 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -511,6 +511,7 @@ void __handle_sysrq(int key, bool check_mask)
>  	int orig_log_level;
>  	int i;
> 
> +	rcu_sysrq_start();
>  	rcu_read_lock();
>  	/*
>  	 * Raise the apparent loglevel to maximum so that the sysrq header
> @@ -554,6 +555,7 @@ void __handle_sysrq(int key, bool check_mask)
>  		console_loglevel = orig_log_level;
>  	}
>  	rcu_read_unlock();
> +	rcu_sysrq_end();
>  }
> 
>  void handle_sysrq(int key)
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 00a7fd6..b9a95f8 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -227,6 +227,8 @@ void rcu_idle_enter(void);
>  void rcu_idle_exit(void);
>  void rcu_irq_enter(void);
>  void rcu_irq_exit(void);
> +void rcu_sysrq_start(void);
> +void rcu_sysrq_end(void);
> 
>  #ifdef CONFIG_RCU_USER_QS
>  void rcu_user_enter(void);
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 4c0a9b0..2ac3289 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -320,6 +320,17 @@ int rcu_jiffies_till_stall_check(void)
>  	return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
>  }
> 
> +void rcu_sysrq_start(void)
> +{
> +	rcu_cpu_stall_suppress = 2;
> +}
> +
> +void rcu_sysrq_end(void)
> +{
> +	if (rcu_cpu_stall_suppress == 2)
> +		rcu_cpu_stall_suppress = 0;
> +}
> +
>  static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
>  {
>  	rcu_cpu_stall_suppress = 1;
>