linux-kernel - Re: [RFC PATCH v1 04/11] sched/idle: make the fast idle path for short idle periods

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Date:   Tue, 11 Jul 2017 18:33:55 +0200
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        peterz@...radead.org, len.brown@...el.com, rjw@...ysocki.net,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, yang.zhang.wz@...il.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [RFC PATCH v1 04/11] sched/idle: make the fast idle path for
 short idle periods

On Tue, Jul 11, 2017 at 05:58:47AM -0700, Paul E. McKenney wrote:
> On Mon, Jul 10, 2017 at 09:38:34AM +0800, Aubrey Li wrote:
> > From: Aubrey Li <aubrey.li@...ux.intel.com>
> > 
> > The system will enter a fast idle loop if the predicted idle period
> > is shorter than the threshold.
> > ---
> >  kernel/sched/idle.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index cf6c11f..16a766c 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -280,6 +280,8 @@ static void cpuidle_generic(void)
> >   */
> >  static void do_idle(void)
> >  {
> > +	unsigned int predicted_idle_us;
> > +	unsigned int short_idle_threshold = jiffies_to_usecs(1) / 2;
> >  	/*
> >  	 * If the arch has a polling bit, we maintain an invariant:
> >  	 *
> > @@ -291,7 +293,12 @@ static void do_idle(void)
> > 
> >  	__current_set_polling();
> > 
> > -	cpuidle_generic();
> > +	predicted_idle_us = cpuidle_predict();
> > +
> > +	if (likely(predicted_idle_us < short_idle_threshold))
> > +		cpuidle_fast();
> 
> What if we get here from nohz_full usermode execution?  In that
> case, if I remember correctly, the scheduling-clock interrupt
> will still be disabled, and would have to be re-enabled before
> we could safely invoke cpuidle_fast().
> 
> Or am I missing something here?

That's a good point. It's partially ok because if the tick is needed
for something specific, it is not entirely stopped but programmed to that
deadline.

Now there is some idle specific code when we enter dynticks-idle. See
tick_nohz_start_idle(), tick_nohz_stop_idle(), sched_clock_idle_wakeup_event()
and some subsystems that react differently when we enter dyntick idle
mode (scheduler_tick_max_deferment) so the tick may need a reevaluation.

For now I'd rather suggest that we treat full nohz as an exception case here
and do:

    if (!tick_nohz_full_cpu(smp_processor_id()) && likely(predicted_idle_us < short_idle_threshold))
        cpuidle_fast();

Ugly but safer!

Thanks.