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

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Date:	Mon, 16 Nov 2015 16:06:57 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Len Brown <len.brown@...el.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry

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
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