Re: Making rcu_normal=1 in RT

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

On Mon, Oct 31, 2016 at 06:38:52PM +0100, Sebastian Andrzej Siewior wrote:
> On 2016-10-16 04:28:46 [-0700], Paul E. McKenney wrote:
> > If the relevant maintainers are OK with that, I am OK with it as long
> > as it is non-default (at least to begin with) and does not introduce
> > additional Kconfig questions.  My guess is that a boot parameter would
> > work best, but something to discuss.
> 
> Okay. For me to summary:
> - we want rcu_normal_after_boot=1 on -RT via CONFIG_PREEMPT_RT_FULL

That would be good.

> - this makes synchronize_rcu_expedited() behave like
>   synchronize_rcu() and therefore I can drop all patches replacing
>   synchronize_rcu_expedited() with synchronize_rcu().

And this would be a benefit.  ;-)

> - optionally it has been requested to make synchronize_rcu() behave like
>   synchronize_rcu_expedited() on shutdown and kexec().

You can invoke rcu_unexpedite_gp() to force all subsequennt grace periods
to be expedited, but you do need to clear rcu_normal for this to have
effect.  Right now, that means "WRITE_ONCE(rcu_normal, 0)", but I could
easily supply a formal API if you would prefer.  Which you probably
would given  that TINY_RCU doesn't have rcu_normal...

> Did I miss understood / forgot something?

It would not hurt to boot with rcu_expedited if boot speed is critical,
but I don't know whether or not this should be enabled by default.

							Thanx, Paul

> Sebastian
>