linux-kernel - [PATCH 0/2] printk, Add a printk.clock kernel parameter

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Date:	Wed,  6 Jan 2016 08:00:32 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Prarit Bhargava <prarit@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Xunlei Pang <pang.xunlei@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Baolin Wang <baolin.wang@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Petr Mladek <pmladek@...e.cz>, Tejun Heo <tj@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Vasily Averin <vvs@...tuozzo.com>,
	Joe Perches <joe@...ches.com>
Subject: [PATCH 0/2] printk, Add a printk.clock kernel parameter

Over the past years I've seen many reports of bugs that include
time-stamped kernel logs (enabled when CONFIG_PRINTK_TIME=y or
print.time=1 is specified as a kernel parameter) that do not align
with either external time stamped logs or /var/log/messages.

For example,

[root@...el-wildcatpass-06 ~]# date; echo "Hello!" > /dev/kmsg ; date
Thu Dec 17 13:58:31 EST 2015
Thu Dec 17 13:58:31 EST 2015

which displays

[83973.768912] Hello!

on the serial console.

Running a script to convert this to "boot time",

[root@...el-wildcatpass-06 ~]# ./human.sh  | tail -1
[Thu Dec 17 13:59:57 2015] Hello!

which is already off by 1 minute and 26 seconds off after ~24 hours of
uptime.

This occurs because the time stamp is obtained from a call to
local_clock() which (on x86) is a direct call to the hardware.  These
hardware clock reads are not modified by the standard ntp or ptp protocol,
while the other timestamps are, and that results in situations where
/var/log/messages timestamps are further and further offset from the
hardware clock timestamps.

This patch introduces printk.clock=[local|boot|real|tai] allowing a
user to specify an adjusted clock to use with printk timestamps.  The
hardware clock, or the existing functionality, is preserved by default.

Cc: John Stultz <john.stultz@...aro.org>
Cc: Xunlei Pang <pang.xunlei@...aro.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Baolin Wang <baolin.wang@...aro.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Petr Mladek <pmladek@...e.cz>
Cc: Tejun Heo <tj@...nel.org>
Cc: Peter Hurley <peter@...leysoftware.com>
Cc: Vasily Averin <vvs@...tuozzo.com>
Cc: Joe Perches <joe@...ches.com>
Signed-off-by: Prarit Bhargava <prarit@...hat.com>

Prarit Bhargava (2):
  kernel, timekeeping, add trylock option to ktime_get_with_offset()
  printk, Add printk.clock kernel parameter

 include/linux/time64.h      |    2 +
 include/linux/timekeeping.h |   50 +++++++++++++++++++--
 kernel/printk/printk.c      |  105 +++++++++++++++++++++++++++++++++++++++----
 kernel/time/timekeeping.c   |   15 ++++++-
 4 files changed, 159 insertions(+), 13 deletions(-)

-- 
1.7.9.3

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