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

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Message-Id: <20170719145130.GC3730@linux.vnet.ibm.com>
Date:   Wed, 19 Jul 2017 07:51:30 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Christoph Lameter <cl@...ux.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        len.brown@...el.com, rjw@...ysocki.net, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, yang.zhang.wz@...il.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On Wed, Jul 19, 2017 at 03:43:07PM +0200, Frederic Weisbecker wrote:
> 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@...radead.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@...radead.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@...radead.org>
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.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.

If you create it, let me know, I could switch to it.

							Thanx, Paul

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