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    SubjectRe: linux-next-20110923: warning kernel/rcutree.c:1833
    On Sat, Oct 01, 2011 at 08:23:16PM -0700, Paul E. McKenney wrote:
    > On Sat, Oct 01, 2011 at 10:07:14AM -0700, Paul E. McKenney wrote:
    > > On Sat, Oct 01, 2011 at 02:24:45PM +0200, Frederic Weisbecker wrote:
    > > > On Fri, Sep 30, 2011 at 12:24:38PM -0700, Paul E. McKenney wrote:
    > > > > And here is a first cut, probably totally broken, but a start.
    > > > >
    > > > > With this change, I am wondering about tick_nohz_stop_sched_tick()'s
    > > > > invocation of rcu_idle_enter() -- this now needs to be called regardless
    > > > > of whether or not tick_nohz_stop_sched_tick() actually stops the tick.
    > > > > Except that if tick_nohz_stop_sched_tick() is invoked with inidle==0,
    > > > > it looks like we should -not- call rcu_idle_enter().
    > > >
    > > > Because of the new check in rcu_check_callbacks()? Yeah.
    > > >
    > > > If you think it's fine to call rcu_enter_nohz() unconditionally
    > > > everytime we enter the idle loop then yeah. I just don't know
    > > > the overhead it adds, as it adds an unconditional tiny piece of
    > > > code before we can finally save the power.
    > > >
    > > > Either entering idle involves extended quiescent state as in this
    > > > patch, or you separate both and then rcu_enter_nohz() is only
    > > > called when the tick is stopped.
    > > >
    > > > If you choose to merge both, you indeed need to call rcu_idle_enter()
    > > > and rcu_idle_exit() whether the tick is stopped or not.
    > > >
    > > > > I eventually just left the rcu_idle_enter() calls in their current
    > > > > places due to paranoia about messing up and ending up with unbalanced
    > > > > rcu_idle_enter() and rcu_idle_exit() calls. Any thoughts on how to
    > > > > make this work better?
    > > >
    > > > Yeah something like this (untested):
    > > >
    > > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
    > > > index d5097c4..ad3ecad 100644
    > > > --- a/kernel/time/tick-sched.c
    > > > +++ b/kernel/time/tick-sched.c
    > > > @@ -273,9 +273,12 @@ void tick_nohz_stop_sched_tick(int inidle)
    > > > * updated. Thus, it must not be called in the event we are called from
    > > > * irq_exit() with the prior state different than idle.
    > > > */
    > > > - if (!inidle && !ts->inidle)
    > > > + if (inidle)
    > > > + rcu_idle_enter();
    > > > + else if (!ts->inidle)
    > > > goto end;
    > > >
    > > > +
    > > > /*
    > > > * Set ts->inidle unconditionally. Even if the system did not
    > > > * switch to NOHZ mode the cpu frequency governers rely on the
    > > > @@ -409,7 +412,6 @@ void tick_nohz_stop_sched_tick(int inidle)
    > > > ts->idle_tick = hrtimer_get_expires(&ts->sched_timer);
    > > > ts->tick_stopped = 1;
    > > > ts->idle_jiffies = last_jiffies;
    > > > - rcu_enter_nohz();
    > > > }
    > > >
    > > > ts->idle_sleeps++;
    > > > @@ -505,6 +507,9 @@ void tick_nohz_restart_sched_tick(void)
    > > > ktime_t now;
    > > >
    > > > local_irq_disable();
    > > > +
    > > > + rcu_idle_exit();
    > > > +
    > > > if (ts->idle_active || (ts->inidle && ts->tick_stopped))
    > > > now = ktime_get();
    > > >
    > > > @@ -519,8 +524,6 @@ void tick_nohz_restart_sched_tick(void)
    > > >
    > > > ts->inidle = 0;
    > > >
    > > > - rcu_exit_nohz();
    > > > -
    > > > /* Update jiffies first */
    > > > select_nohz_load_balancer(0);
    > > > tick_do_update_jiffies64(now);
    > > >
    > > >
    > > >
    > > >
    > > > More things about your patch below:
    > > >
    > > > > --- a/kernel/rcutiny.c
    > > > > +++ b/kernel/rcutiny.c
    > > > > @@ -54,31 +54,47 @@ static void __call_rcu(struct rcu_head *head,
    > > > >
    > > > > #include "rcutiny_plugin.h"
    > > > >
    > > > > -#ifdef CONFIG_NO_HZ
    > > > > -
    > > > > static long rcu_dynticks_nesting = 1;
    > > > >
    > > > > /*
    > > > > - * Enter dynticks-idle mode, which is an extended quiescent state
    > > > > - * if we have fully entered that mode (i.e., if the new value of
    > > > > - * dynticks_nesting is zero).
    > > > > + * Enter idle, which is an extended quiescent state if we have fully
    > > > > + * entered that mode (i.e., if the new value of dynticks_nesting is zero).
    > > > > */
    > > > > -void rcu_enter_nohz(void)
    > > > > +void rcu_idle_enter(void)
    > > > > {
    > > > > if (--rcu_dynticks_nesting == 0)
    > > > > rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
    > > > > }
    > > > >
    > > > > /*
    > > > > - * Exit dynticks-idle mode, so that we are no longer in an extended
    > > > > - * quiescent state.
    > > > > + * Exit idle, so that we are no longer in an extended quiescent state.
    > > > > */
    > > > > -void rcu_exit_nohz(void)
    > > > > +void rcu_idle_exit(void)
    > > > > {
    > > > > rcu_dynticks_nesting++;
    > > > > }
    > > > >
    > > > > -#endif /* #ifdef CONFIG_NO_HZ */
    > > > > +#ifdef CONFIG_PROVE_RCU
    > > > > +
    > > > > +/*
    > > > > + * Test whether the current CPU is idle.
    > > > > + */
    > > >
    > > > Is idle from an RCU point of view yeah.
    > >
    > > Good point -- I now say "Test whether RCU thinks that the current CPU is idle."
    > >
    > > > > +int rcu_is_cpu_idle(void)
    > > > > +{
    > > > > + return !rcu_dynticks_nesting;
    > > > > +}
    > > > > +
    > > > > +#endif /* #ifdef CONFIG_PROVE_RCU */
    > > > > +
    > > > > +/*
    > > > > + * Test whether the current CPU was interrupted from idle. Nested
    > > > > + * interrupts don't count, we must be running at the first interrupt
    > > > > + * level.
    > > > > + */
    > > > > +int rcu_is_cpu_rrupt_from_idle(void)
    > > > > +{
    > > > > + return rcu_dynticks_nesting <= 0;
    > > > > +}
    > > > >
    > > > > /*
    > > > > * Helper function for rcu_sched_qs() and rcu_bh_qs().
    > > > > @@ -131,10 +147,7 @@ void rcu_bh_qs(int cpu)
    > > > > */
    > > > > void rcu_check_callbacks(int cpu, int user)
    > > > > {
    > > > > - if (user ||
    > > > > - (idle_cpu(cpu) &&
    > > > > - !in_softirq() &&
    > > > > - hardirq_count() <= (1 << HARDIRQ_SHIFT)))
    > > > > + if (user || rcu_is_cpu_rrupt_from_idle())
    > > > > rcu_sched_qs(cpu);
    > > >
    > > > It wasn't obvious to me in the first shot. This might need a comment
    > > > that tells rcu_check_callbacks() is called from an interrupt
    > > > and thus need to handle that first level in the check.
    > >
    > > OK, I added "This function must be called from hardirq context".
    > >
    > > > Other than that, looks good overall.
    > >
    > > Keeping fingers firmly crossed for the testing...
    >
    > And it appears sane in testing thus far. I have consolidated to one
    > patch and pushed to https://github.com/paulmckrcu/linux branch rcu/dev.
    >
    > Testing continues.

    Great. I'll start the rebase then.

    Thanks.


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