linux-kernel - Re: [PATCH 0/2] printk, Add printk.clock kernel parameter [v2]

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Message-ID: <alpine.DEB.2.11.1601141543130.3575@nanos>
Date:	Thu, 14 Jan 2016 15:44:23 +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 0/2] printk, Add printk.clock kernel parameter [v2]

On Thu, 14 Jan 2016, Petr Mladek wrote:
> On Wed 2016-01-13 18:28:50, Thomas Gleixner wrote:
> > You can solve the whole business by changing the timestamp in printk_log to
> > 
> >    u64	    mono;
> >    u64	    offset_real;
> 
> This is not so easy because the structure is proceed by userspace tool,
> e.g. crash, see log_buf_kexec_setup(). We would need to update all
> the tools as well.

Fair enough.
 
> > and have a function which does:
> > 
> > u64 ktime_get_log_ts(u64 *offset_real)
> > {
> > 	*offset_real = tk_core.timekeeper.offs_real;
> > 
> > 	if (timekeeping_active)
> > 		return ktime_get_mono_fast_ns();
> > 	else
> > 		return local_clock();
> > }
> 
> A solution would be to apply the offset_real immediately. I wonder if
> any tool expects the messages to be sorted by a monotonic clock. In
> fact, it might be useful to see that some messages are disordered
> against the real time, e.g. because of the leaf second.

Not only leap seconds, it's also settimeofday and NTP might make the wall time
jump under certain conditions.

Thanks,

	tglx