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

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

On Wed, Aug 22, 2012 at 07:35:45PM +0200, Frederic Weisbecker wrote:
> On Wed, Aug 22, 2012 at 10:19:30AM -0700, Paul E. McKenney wrote:
> > On Wed, Aug 22, 2012 at 06:23:39PM +0200, Frederic Weisbecker wrote:
> > > In the old times, the whole idle task was considered
> > > as an RCU quiescent state. But as RCU became more and
> > > more successful overtime, some RCU read side critical
> > > section have been added even in the code of some
> > > architectures idle tasks, for tracing for example.
> > > 
> > > So nowadays, rcu_idle_enter() and rcu_idle_exit() must
> > > be called by the architecture to tell RCU about the part
> > > in the idle loop that doesn't make use of rcu read side
> > > critical sections, typically the part that puts the CPU
> > > in low power mode.
> > > 
> > > This is necessary for RCU to find the quiescent states in
> > > idle in order to complete grace periods.
> > > 
> > > Add this missing pair of calls in the Alpha's idle loop.
> > > 
> > > Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > > Cc: Richard Henderson <rth@twiddle.net>
> > > Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
> > > Cc: Matt Turner <mattst88@gmail.com>
> > > Cc: alpha <linux-alpha@vger.kernel.org>
> > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Cc: 3.2.x.. <stable@kernel.org>
> > > ---
> > >  arch/alpha/kernel/process.c |    6 +++++-
> > >  1 files changed, 5 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/arch/alpha/kernel/process.c b/arch/alpha/kernel/process.c
> > > index 153d3fc..2ebf7b5 100644
> > > --- a/arch/alpha/kernel/process.c
> > > +++ b/arch/alpha/kernel/process.c
> > > @@ -28,6 +28,7 @@
> > >  #include <linux/tty.h>
> > >  #include <linux/console.h>
> > >  #include <linux/slab.h>
> > > +#include <linux/rcupdate.h>
> > > 
> > >  #include <asm/reg.h>
> > >  #include <asm/uaccess.h>
> > > @@ -50,13 +51,16 @@ cpu_idle(void)
> > >  {
> > >  	set_thread_flag(TIF_POLLING_NRFLAG);
> > > 
> > > +	preempt_disable();
> > 
> > I don't understand the above preempt_disable() not having a matching
> > preempt_enable() at exit, but the rest of the patches in this series
> > look good to me.
> 
> 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.

							Thanx, Paul

> > >  }
> > > 
> > > -- 
> > > 1.7.5.4
> > > 
> > 
>