Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

[Frederic Weisbecker <fweisbec@gmail.com>]

On Wed, Jul 12, 2017 at 08:56:51AM -0700, Paul E. McKenney wrote:
> On Wed, Jul 12, 2017 at 01:54:51PM +0200, Peter Zijlstra wrote:
> > On Tue, Jul 11, 2017 at 11:09:31AM -0700, Paul E. McKenney wrote:
> > > On Tue, Jul 11, 2017 at 06:34:22PM +0200, Peter Zijlstra wrote:
> > 
> > > > But I think we can at the very least do this; it only gets called from
> > > > kernel/sched/idle.c and both callsites have IRQs explicitly disabled by
> > > > that point.
> > > > 
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 51d4c3acf32d..dccf2dc8155a 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -843,13 +843,8 @@ static void rcu_eqs_enter(bool user)
> > > >   */
> > > >  void rcu_idle_enter(void)
> > > >  {
> > > > -	unsigned long flags;
> > > > -
> > > > -	local_irq_save(flags);
> > > 
> > > With this addition, I am all for it:
> > > 
> > > RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_idle_enter() invoked with irqs enabled!!!");
> > > 
> > > If you are OK with this addition, may I please have your Signed-off-by?
> > 
> > Sure,
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> 
> Very good, I have queued the patch below.  I left out the removal of
> the export as I need to work out why the export was there.  If it turns
> out not to be needed, I will remove the related ones as well.
> 
> Fair enough?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 95f3e587ce6388028a51f0c852800fca944e7032
> Author: Peter Zijlstra (Intel) <peterz@infradead.org>
> Date:   Wed Jul 12 07:59:54 2017 -0700
> 
>     rcu: Make rcu_idle_enter() rely on callers disabling irqs
>     
>     All callers to rcu_idle_enter() have irqs disabled, so there is no
>     point in rcu_idle_enter disabling them again.  This commit therefore
>     replaces the irq disabling with a RCU_LOCKDEP_WARN().
>     
>     Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 6de6b1c5ee53..c78b076653ce 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -840,11 +840,8 @@ static void rcu_eqs_enter(bool user)
>   */
>  void rcu_idle_enter(void)
>  {
> -	unsigned long flags;
> -
> -	local_irq_save(flags);
> +	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_idle_enter() invoked with irqs enabled!!!");

Nice! I really need to make that LOCKDEP_ASSERT_IRQS_DISABLED() patchset because there
are many places where either we have WARN_ON_ONCE(!irqs_disabled()) or we need it but we were too
shy to do it for performance reason.

>  	rcu_eqs_enter(false);
> -	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(rcu_idle_enter);
>  
>