On Fri, Aug 09, 2013 at 06:20:59PM +0200, Frederic Weisbecker wrote:
> On Fri, Jul 26, 2013 at 04:19:23PM -0700, Paul E. McKenney wrote:
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index 3edae39..ff84bed 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -28,7 +28,7 @@
> > #include <linux/gfp.h>
> > #include <linux/oom.h>
> > #include <linux/smpboot.h>
> > -#include <linux/tick.h>
> > +#include "time/tick-internal.h"
> >
> > #define RCU_KTHREAD_PRIO 1
> >
> > @@ -2395,12 +2395,12 @@ static void rcu_kick_nohz_cpu(int cpu)
> > * most active flavor of RCU.
> > */
> > #ifdef CONFIG_PREEMPT_RCU
> > -static struct rcu_state __maybe_unused *rcu_sysidle_state =
> > &rcu_preempt_state;
> > +static struct rcu_state *rcu_sysidle_state = &rcu_preempt_state;
> > #else /* #ifdef CONFIG_PREEMPT_RCU */
> > -static struct rcu_state __maybe_unused *rcu_sysidle_state =
> > &rcu_sched_state;
> > +static struct rcu_state *rcu_sysidle_state = &rcu_sched_state;
> > #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
>
> Ah you fixed it here. Ok :)
Bisectability and all that. ;-)
> > -static int __maybe_unused full_sysidle_state; /* Current system-idle
> > state. */
> > +static int full_sysidle_state; /* Current system-idle state. */
> > #define RCU_SYSIDLE_NOT 0 /* Some CPU is not idle. */
> > #define RCU_SYSIDLE_SHORT 1 /* All CPUs idle for brief period. */
> > #define RCU_SYSIDLE_LONG 2 /* All CPUs idle for long enough. */
> [...]
> > +/*
> > + * Check to see if the system is fully idle, other than the timekeeping
> > CPU.
> > + * The caller must have disabled interrupts.
> > + */
> > +bool rcu_sys_is_idle(void)
> > +{
> > + static struct rcu_sysidle_head rsh;
> > + int rss = ACCESS_ONCE(full_sysidle_state);
> > +
> > + if (WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu))
> > + return false;
> > +
> > + /* Handle small-system case by doing a full scan of CPUs. */
> > + if (nr_cpu_ids <= RCU_SYSIDLE_SMALL) {
>
> I don't understand how the nr_cpu_ids > RCU_SYSIDLE_SMALL is handled. There
> don't
> seem to be other calls of rcu_sysidle_check_cpu() than for small systems.
The other calls are from kernel/rcutree.c from the force-quiescent-state
code. If we have a big system, we don't check until we have some other
reason to touch the cache lines. If we have a small system, we just
dig through them on transition to idle.
Thanx, Paul
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