On Tue, May 27, 2014 at 07:30:50PM -0400, Steven Rostedt wrote:
> On Tue, 27 May 2014 15:21:39 -0700
> Stephen Boyd <sb...@codeaurora.org> wrote:
>
>
> > > Arnd brings up a good point.
> >
> > Hrm.. still not getting Arnd's mails.
>
> Strange. What mail service do you have. Could they be blocking him?
>
> >
> > > If we disable irqs off tracing completely,
> > > we may be missing places in the idle path that disable interrupts for
> > > long periods of time. We may want to move the stop down further.
> > >
> > > The way it works (IIRC), and why tracing can start again is that it can
> > > nest. Perhaps we need to stop it further down if we can't move it
> > > completely.
> > >
> >
> > I'm not sure how much deeper it can go and I'm afraid it will become a
> > game of whack-a-mole. I already see two places that disable and reenable
> > irqs after stop_critical_timings() is called (first in rcu_idle_enter()
> > and second in clockevents_notify()). Should rcu_idle_enter() move to
> > raw_local_irq_save()? It looks like that path calls rcu_sched_qs() and
> > on tiny RCU that again needs the raw_ treatement. We can probably call
> > stop_critical_timings() after rcu_idle_enter() to fix this.
>
> I don't think we need to whack-a-mole. The start stop should be around
> where it goes to sleep.
>
> >
> > What about clockevents_notify? __raw_spin_lock_irqsave() should probably
> > use raw_local_irqsave().
>
> No that solution is even worse. We need lockdep working here.
>
> >
> > If we go the raw route, do we even need stop/start_critical_timings()?
> > Can't we just use raw accessors in the idle paths
> > (tick_nohz_idle_{enter,exit}(), cpuidle_enter(), etc.) and get rid of
> > the stop/start stuff completely? I admit this patch is pretty much a big
> > sledge hammer that tries to make things simple, but if there is some
> > benefit to the raw accessors I'm willing to send patches to fix all the
> > call sites.
> >
>
> How about the following. I don't see any reason stop_critical_timings()
> can't be called from within rcu_idle code, as it doesn't use any rcu.
>
> Paul, Peter, see anything wrong with this?
I don't immediately see any RCU use by stop_critical_timings(), but could
easily have missed something. But CONFIG_PROVE_RCU=y will normally yell
if something using RCU showed up.
Looks plausible, but clearly needs testing across the usual array of
configs and arches.
Thanx, Paul
> -- Steve
>
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 8f4390a..f5e6a64 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -88,12 +88,6 @@ static int cpuidle_idle_call(void)
> }
>
> /*
> - * During the idle period, stop measuring the disabled irqs
> - * critical sections latencies
> - */
> - stop_critical_timings();
> -
> - /*
> * Tell the RCU framework we are entering an idle section,
> * so no more rcu read side critical sections and one more
> * step to the grace period
> @@ -144,6 +138,12 @@ static int cpuidle_idle_call(void)
> trace_cpu_idle_rcuidle(next_state, dev->cpu);
>
> /*
> + * During the idle period, stop measuring the
> + * disabled irqs critical sections latencies
> + */
> + stop_critical_timings();
> +
> + /*
> * Enter the idle state previously
> * returned by the governor
> * decision. This function will block
> @@ -154,6 +154,8 @@ static int cpuidle_idle_call(void)
> entered_state = cpuidle_enter(drv, dev,
> next_state);
>
> + start_critical_timings();
> +
> trace_cpu_idle_rcuidle(PWR_EVENT_EXIT,
> dev->cpu);
>
> @@ -175,8 +177,11 @@ static int cpuidle_idle_call(void)
> * We can't use the cpuidle framework, let's use the default
> * idle routine
> */
> - if (ret)
> + if (ret) {
> + stop_critical_timings();
> arch_cpu_idle();
> + start_critical_timings();
> + }
>
> __current_set_polling();
>
> @@ -188,7 +193,6 @@ static int cpuidle_idle_call(void)
> local_irq_enable();
>
> rcu_idle_exit();
> - start_critical_timings();
>
> return 0;
> }
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
