Re: Thoughts on this RCU idle entry/exit patch?

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

On Tue, Oct 08, 2013 at 10:34:28PM +0200, Frederic Weisbecker wrote:
> On Mon, Oct 07, 2013 at 08:39:55AM -0700, Paul E. McKenney wrote:
> > Hello, Frederic!
> > 
> > The following patch seems to me to be a good idea to better handle
> > task nesting.  Any reason why it would be a bad thing?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > rcu: Allow task-level idle entry/exit nesting
> > 
> > The current task-level idle entry/exit code forces an entry/exit on
> > each call, regardless of the nesting level.  This commit therefore
> > properly accounts for nesting.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Looks good. In fact, the current code is even buggy because two nesting rcu_user_eqs()
> as in:
> 
>         rcu_eqs_enter()
>             rcu_eqs_enter()
>             rcu_eqs_exit()
>         rcu_eqs_exit()
> 
> would result in rdtp->dynticks wrong increment, right?

That was my thought, but I figured I should run it past you in case
there was some subtle tie-in to NO_HZ_FULL.

> So that's even a bug fix. I wonder if it's a regression. That said rcu_eqs_enter_common()
> should warn on such miscount, so may be these functions actually don't nest in practice
> or you would have received such warnings.

And the lack of such warnings was another reason I felt the need to check
with you.

> So I wonder, do we want to continue to allow this nesting? I remember that DYNTICK_TASK_NEST_*
> stuff is there to protects against non finishing interrupts on some archs (I also remember that
> this, or at least a practical scenario for this, was hard to really define though :o)
> But then wouldn't it involve other kind of scenario like this?
> 
>        rcu_irq_enter()
>            rcu_eqs_enter()
>            rcu_eqs_exit()
>            ...
> 
> Anyway, that's just random thougths on further simplifications, in any case, this
> patch looks good.

Yep, if no task-level nesting is ever required, things could be a bit
simpler.  I would be a bit slow about making such a change, though.
After all, the need to deal with Hotel California interrupts means that
handling nesting isn't that big of a deal comparatively.  ;-)

May I add your Reviewed-by?

							Thanx, Paul

> Thanks.
> 
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 106f7f5cdd1d..f0be20886617 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -411,11 +411,12 @@ static void rcu_eqs_enter(bool user)
> >  	rdtp = this_cpu_ptr(&rcu_dynticks);
> >  	oldval = rdtp->dynticks_nesting;
> >  	WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
> > -	if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE)
> > +	if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE) {
> >  		rdtp->dynticks_nesting = 0;
> > -	else
> > +		rcu_eqs_enter_common(rdtp, oldval, user);
> > +	} else {
> >  		rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
> > -	rcu_eqs_enter_common(rdtp, oldval, user);
> > +	}
> >  }
> >  
> >  /**
> > @@ -533,11 +534,12 @@ static void rcu_eqs_exit(bool user)
> >  	rdtp = this_cpu_ptr(&rcu_dynticks);
> >  	oldval = rdtp->dynticks_nesting;
> >  	WARN_ON_ONCE(oldval < 0);
> > -	if (oldval & DYNTICK_TASK_NEST_MASK)
> > +	if (oldval & DYNTICK_TASK_NEST_MASK) {
> >  		rdtp->dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
> > -	else
> > +	} else {
> >  		rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
> > -	rcu_eqs_exit_common(rdtp, oldval, user);
> > +		rcu_eqs_exit_common(rdtp, oldval, user);
> > +	}
> >  }
> >  
> >  /**
> > 
>