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

Thomas Gleixner
2015-11-16T15:07:48Z

On Fri, 13 Nov 2015, Jacob Pan wrote:
> <idle>-0 [000] 30.093473: bprint: __tick_nohz_idle_enter: JPAN: __tick_nohz_idle_enter 803
> <idle>-0 [000] 30.093473: bprint: __tick_nohz_idle_enter: JPAN: can_stop_idle_tick 743
> [JP] can_stop_idle_tick() checks ok to stop tick
>
> <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?

Thanks,

tglx