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

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

On Fri, Dec 14, 2018 at 12:31:11PM -0800, Andrew Morton wrote:
> On Sat, 15 Dec 2018 00:17:38 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> 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.
> > 
> > --- 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)
> 
> This won't work correctly if rcu_cpu_stall_timeout is set to something
> stupidly small.  Perhaps is would be better to make this code aware of
> the current rcu_cpu_stall_timeout setting?

Good point.

However, the reason that I wasn't worried because any settings of
rcu_cpu_stall_timeout less than 3 seconds are cheerfully bumped up to
3 seconds, so we have a safety factor of 30 as things stand.

I could export the minimum, though, if that would be helpful.

							Thanx, Paul