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();
> > }
> > }
> >
> > --
> > 1.7.5.4
> >
>
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