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

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Date:	Wed, 27 Jan 2016 19:31:57 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <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

2016-01-26 23:21 GMT+08:00 Thomas Gleixner <tglx@...utronix.de>:
> 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;

Yeah, I know this catch it and that's why no real warp happen, just send out v2,
thanks for your review, Thomas. :-)

Regards,
Wanpeng Li