Re: [PATCH] kernel/hung_task.c: Break RCU locks based on jiffies.

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

On Sat, Dec 15, 2018 at 12:17:38AM +0900, Tetsuo Handa wrote:
> check_hung_uninterruptible_tasks() is currently calling rcu_lock_break()
> for every 1024 threads. But check_hung_task() is very slow if printk()
> was called, and is very fast otherwise. If many threads within some 1024
> threads called printk(), the RCU grace period might be extended enough
> to trigger RCU stall warnings. Therefore, calling rcu_lock_break() for
> every some fixed jiffies will be safer.
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

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

> ---
>  kernel/hung_task.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index cb8e3e8..444b8b5 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -34,7 +34,7 @@
>   * is disabled during the critical section. It also controls the size of
>   * the RCU grace period. So it needs to be upper-bound.
>   */
> -#define HUNG_TASK_BATCHING 1024
> +#define HUNG_TASK_LOCK_BREAK (HZ / 10)
> 
>  /*
>   * Zero means infinite timeout - no checking done:
> @@ -173,7 +173,7 @@ static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
>  static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  {
>  	int max_count = sysctl_hung_task_check_count;
> -	int batch_count = HUNG_TASK_BATCHING;
> +	unsigned long last_break = jiffies;
>  	struct task_struct *g, *t;
> 
>  	/*
> @@ -188,10 +188,10 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  	for_each_process_thread(g, t) {
>  		if (!max_count--)
>  			goto unlock;
> -		if (!--batch_count) {
> -			batch_count = HUNG_TASK_BATCHING;
> +		if (time_after(jiffies, last_break + HUNG_TASK_LOCK_BREAK)) {
>  			if (!rcu_lock_break(g, t))
>  				goto unlock;
> +			last_break = jiffies;
>  		}
>  		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
>  		if (t->state == TASK_UNINTERRUPTIBLE)
> -- 
> 1.8.3.1
>