linux-kernel - Re: [PATCH] nohz: Fix collision between tick and other hrtimers

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Date:   Thu, 29 Dec 2016 17:53:53 +0100
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        James Hartsock <hartsjc@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] nohz: Fix collision between tick and other hrtimers

On Thu, Dec 29, 2016 at 05:42:48PM +0100, Thomas Gleixner wrote:
> On Sat, 24 Dec 2016, Frederic Weisbecker wrote:
> >  static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >  					 ktime_t now, int cpu)
> >  {
> > -	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
> >  	u64 basemono, next_tick, next_tmr, next_rcu, delta, expires;
> >  	unsigned long seq, basejiff;
> >  	ktime_t	tick;
> > @@ -767,7 +766,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >  	tick.tv64 = expires;
> >  
> >  	/* Skip reprogram of event if its not changed */
> > -	if (ts->tick_stopped && (expires == dev->next_event.tv64))
> > +	if (ts->tick_stopped && (expires == ts->next_tick.tv64))
> >  		goto out;
> 
> Good catch!
> 
> >  
> >  	/*
> > @@ -787,6 +786,8 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >  		trace_tick_stop(1, TICK_DEP_MASK_NONE);
> >  	}
> >  
> > +	ts->next_tick = tick;
> > +
> >  	/*
> >  	 * If the expiration time == KTIME_MAX, then we simply stop
> >  	 * the tick timer.
> > @@ -803,7 +804,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >  		tick_program_event(tick, 1);
> >  out:
> >  	/* Update the estimated sleep length */
> > -	ts->sleep_length = ktime_sub(dev->next_event, now);
> > +	ts->sleep_length = ktime_sub(ts->next_tick, now);
> 
> This is wrong. If the next event is earlier than the next estimated tick
> then tick_nohz_get_sleep_length() will return crap and the idle governor
> will go into a deeper C-state than sensible.

Ah I see, the governor wants to know about the next timer, whether it is the tick
or not, right? I'll fix that and improve the comment along.

Thanks.