linux-kernel - Re: [PATCH 01/10] alpha: Add missing RCU idle APIs on idle loop

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Message-ID: <20120823104201.GA18251@somewhere.redhat.com>
Date:	Thu, 23 Aug 2012 12:42:11 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Matt Turner <mattst88@...il.com>,
	alpha <linux-alpha@...r.kernel.org>,
	"3.2.x.." <stable@...nel.org>
Subject: Re: [PATCH 01/10] alpha: Add missing RCU idle APIs on idle loop

On Wed, Aug 22, 2012 at 12:01:09PM -0700, Paul E. McKenney wrote:
> > The current code is preemptable, at least it appears so because it calls
> > schedule() directly. And if I call rcu_idle_enter() in a preemptable section,
> > I'm in trouble because I'll schedule while in extended QS.
> > 
> > Thus I need to disable preemption here at least until I call rcu_idle_exit().
> > 
> > Now this is an endless loop so there is no need to re-enable
> > preemption after the loop. And schedule_preempt_disabled()
> > takes care of enabling preemption before schedule() and redisabling
> > it afterward.
> > 
> > 
> > > 
> > > 							Thanx, Paul
> > > 
> > > >  	while (1) {
> > > >  		/* FIXME -- EV6 and LCA45 know how to power down
> > > >  		   the CPU.  */
> > > > 
> > > > +		rcu_idle_enter();
> > > >  		while (!need_resched())
> > > >  			cpu_relax();
> > > > -		schedule();
> > > > +		rcu_idle_exit();
> > > > +		schedule_preempt_disabled();
> > > >  	}
> 
> Understood, but what I don't understand is why you don't need a
> preempt_enable() right here.

Look, let's inline the content of schedule_preempt_disabled(), the code
then looks like:

void cpu_idle(void)
{
	set_thread_flag(TIF_POLLING_NRFLAG);

	preempt_disable();
	while (1) {
		/* FIXME -- EV6 and LCA45 know how to power down
		   the CPU.  */

		rcu_idle_enter();
		while (!need_resched())
			cpu_relax();
		rcu_idle_exit();

		sched_preempt_enable_no_resched();
	        schedule();
	        preempt_disable();
	}
}

So there is a preempt_enable() before we schedule, then we re-disable
preemption after schedule.

Now I realize cpu_idle() is supposed to be called with preemption disabled
already so I shouldn't add an explicit preempt_disable() or it's going to be worse.
But that means there is an existing bug here in alpha, it should call schedule_preempt_disabled()
instead of schedule(). cpu_idle() is called with preemption disabled on the boot CPU.
And it should as well from the secondary CPUs entry but alpha doesn't seem to do that.

So I need to fix that first. I'll respin.

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