linux-kernel - Re: [PATCH] printk, allow different timestamps for printk.time [v2]

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Date:	Thu, 4 Feb 2016 18:25:29 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Petr Mladek <pmladek@...e.com>
cc:	Prarit Bhargava <prarit@...hat.com>, linux-kernel@...r.kernel.org,
	John Stultz <john.stultz@...aro.org>,
	Xunlei Pang <pang.xunlei@...aro.org>,
	Baolin Wang <baolin.wang@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tejun Heo <tj@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Vasily Averin <vvs@...tuozzo.com>,
	Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] printk, allow different timestamps for printk.time
 [v2]

On Thu, 4 Feb 2016, Petr Mladek wrote:
> On Thu 2016-01-28 07:43:49, Prarit Bhargava wrote:
> > +static u64 printk_get_ts(void)
> > +{
> > +	u64 mono, offset_real;
> > +
> > +	if (printk_time == 0)
> > +		return 0;
> > +
> > +	if (printk_time == 1)
> > +		return local_clock();
> > +
> > +	mono = ktime_get_log_ts(&offset_real);
> > +
> > +	if (printk_time == 2)
> > +		return mono;
> > +
> > +	return mono + offset_real;
> 
> At least dmesg is not capable to read the absolute size of the
> real time. It expects offset against the start of the timekeeping
> stuff or so. I get this:
> 
> $> dmesg | tail -n 5
> [    7.128924] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
> [    0.000000] printk: timestamp set to 0.
> [  179.983704] printk: timestamp set to 1.
> [  181.895655] printk: timestamp set to 2.
> [1454602412.026424] printk: timestamp set to 3.
> 
> $dmesg -T -S | tail -n 5
> [Thu Feb  4 17:10:34 2016] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
> [Thu Feb  4 17:10:27 2016] printk: timestamp set to 0.
> [Thu Feb  4 17:13:26 2016] printk: timestamp set to 1.
> [Thu Feb  4 17:13:28 2016] printk: timestamp set to 2.
> [Fri Mar 10 09:23:59 2062] printk: timestamp set to 3.
> 
> Please, note that the last entry points to the year 2062.

That's a problem of dmesg -T -S. It doesn't know that the timestamp (3) is
actually CLOCKTIME_REAL already:

-> 1454602412 / (3600 * 24 * 365)
 = 46.1251
-> 1970 + 46
 = 2016
 
Thanks,

	tglx