Re: [PATCH 2/4] timer: relax tick stop in idle entry

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Nov 16, 2015 at 01:51:26PM -0800, Jacob Pan wrote:
> On Mon, 16 Nov 2015 16:06:57 +0100 (CET)
> Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > >           <idle>-0     [000]    30.093474: bprint:
> > > __tick_nohz_idle_enter: JPAN: tick_nohz_stop_sched_tick 609 delta
> > > 1000000 [JP] but sees delta is exactly 1 tick away. didn't stop
> > > tick.  
> > 
> > If the delta is 1 tick then it is not supposed to stop it. Did you
> > ever try to figure out WHY it is 1 tick?
> > 
> > There are two code pathes which can set it to basemono + TICK_NSEC:
> > 
> >         if (rcu_needs_cpu(basemono, &next_rcu) ||
> >             arch_needs_cpu() || irq_work_needs_cpu()) {
> >                 next_tick = basemono + TICK_NSEC;
> >         } else {
> >                 next_tmr = get_next_timer_interrupt(basejiff,
> > basemono); ts->next_timer = next_tmr;
> >                 /* Take the next rcu event into account */
> >                 next_tick = next_rcu < next_tmr ? next_rcu : next_tmr;
> >         }
> > 
> > Can you please figure out WHY the tick is requested to continue
> > instead of blindly wreckaging the logic in that code?
> 
> Looks like the it hits in both cases during forced idle.
> + Josh
> + Paul
> 
> For the first case, it is always related to RCU. I found there are two
> CONFIG options to avoid this undesired tick in idle loop.
> 1. enable CONFIG_RCU_NOCB_CPU_ALL, offload to orcu kthreads
> 2. or enable CONFIG_RCU_FAST_NO_HZ (enter dytick idle w/ rcu callback)
> 
> Either one works but my concern is that users may not realize the
> intricate CONFIG_ options and how they translate into energy savings.
> Consulted with Josh, it seems we could add a check here to recognize
> the forced idle state and relax rcu_needs_cpu() to return false even it
> has callbacks. Since we are blocking everybody for a short time (5 ticks
> default). It should not impact synchronize and kfree rcu.

Right; as long as you're blocking *everybody*, and RCU priority boosting
doesn't come into play (meaning a real-time task is waiting on RCU
callbacks), then I don't see any harm in blocking RCU callbacks for a
while.  You'd block completion of synchronize_rcu() and similar, as well
as memory reclamation, but since you've blocked *every* CPU systemwide
then that doesn't cause a problem.