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

[Jacob Pan]

On Mon, 16 Nov 2015 16:06:57 +0100 (CET)
Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.

For the second case, which is much more rare, I think we do have next
timer exactly one tick away. Just don't know why tick will continue into
idle loop.
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