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@xxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> > > Cc: Richard Henderson <rth@xxxxxxxxxxx>
> > > Cc: Ivan Kokshaysky <ink@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Matt Turner <mattst88@xxxxxxxxx>
> > > Cc: alpha <linux-alpha@xxxxxxxxxxxxxxx>
> > > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > Cc: 3.2.x.. <stable@xxxxxxxxxx>
> > > ---
> > > 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
> > >
> >
>
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