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 11:11:08 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Frederic Weisbecker <fweisbec@...il.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 06:33:55PM +0200, Frederic Weisbecker wrote:
> 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!

Works for me!

							Thanx, Paul