linux-kernel - Re: [PATCH] nohz: fix jiffies warp while switch to low-res nohz mode

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Date:	Tue, 26 Jan 2016 16:21:01 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Wanpeng Li <kernellwp@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] nohz: fix jiffies warp while switch to low-res nohz
 mode

On Thu, 21 Jan 2016, Wanpeng Li wrote:

> After commit 0ff53d096422 ("tick: sched: Force tick interrupt and get rid 
> of softirq magic"), next expire time is overwritten by last_jiffies_update 
> while switch to low-res nohz mode which leads to jiffies warp. This patch 
> fix it by programming ce device the expire time in the future.

> -	hrtimer_forward_now(&ts->sched_timer, tick_period);
>  	hrtimer_set_expires(&ts->sched_timer, next);
> -	tick_program_event(next, 1);
> +	hrtimer_forward_now(&ts->sched_timer, tick_period);
> +	tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
>  	tick_nohz_activate(ts, NOHZ_MODE_LOWRES);

I agree, that the ordering forward/set is wrong and needs to be fixed, but how
does that result in a jiffies warp?

tick_nohz_handler()
  tick_sched_do_timer()
    tick_do_update_jiffies64()
        delta = ktime_sub(now, last_jiffies_update);
        if (delta.tv64 < tick_period.tv64)
                return;

So if that timer fires early, tick_do_update_jiffies64() will just return and
do nothing. So what makes jiffies warp?

Unless I'm missing something, this changelog is patently wrong.

Thanks,

	tglx