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

Paul E. McKenney
2017-07-12T15:58:24Z

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!!!");
rcu_eqs_enter(false);
- local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(rcu_idle_enter);